The Biblical Unitarian view on this is fairly straightforward. Although Jesus can and ought to be worshipped as King, God is a larger ultimate object of worship. So the angel mentioned God and not Jesus.

See Should we "worship" Jesus Christ? which lays out the issue of worship and Jesus from a Biblical Unitarian perspective.

When John “fell down to worship at the feet of the angel who showed” these visions to him, the angel said, “You must not do that! I am a fellow servant … Worship God” (Rev 22:8-9). The question is, why didn't the angel tell John to worship both God and Jesus?

From a Trinitarian perspective, a possible answer may be that “God” includes Jesus. However, that is not how the word “God” is used in Revelation. The title "God" is found about 100 times in Revelation. The title "God" is found about 100 times in Revelation. In most instances, nobody else is mentioned in the context so it is not immediately clear to whom the title "God" refers. However, in the following 17 instances, the title is used to identify the Father in distinction from Jesus, making it clear that Jesus is NEVER called God and that the title "God" ALWAYS refers exclusively to the Father. The point of the following is not to show that the Father and Son are different Persons, but that GOD AND THE SON ARE DIFFERENT PERSONS:

“The Revelation of Jesus Christ, which God gave Him” (Rev 1:1).

“John, who testified to the word of God and to the testimony of Jesus Christ” (Rev 1:2)

“I, John … was on the island called Patmos because of the word of God and the testimony of Jesus” (Rev 1:9).

“You (the Lamb – Jesus) were slain, and purchased for God with Your blood men from every tribe …” (Rev 5:9).

“Salvation to our God who sits on the throne, and to the Lamb” (Rev 7:10).

“The Lamb … will be their shepherd … and God will wipe every tear from their eyes” (Rev 7:17).

“She gave birth to a son, a male child, who is to rule all the nations with a rod of iron; and her child was caught up to God and to His throne” (Rev 12:5).

“Now … the kingdom of our God and the authority of His Christ have come” (Rev 12:10).

“The dragon … went off to make war with the rest of her children, who keep the commandments of God and hold to the testimony of Jesus” (Rev 12:17).

“These have been purchased from among men as first fruits to God and to the Lamb” (Rev 14:4).

“The saints who keep the commandments of God and their faith in Jesus” (Rev 14:12)

“Those who had been beheaded because of their testimony of Jesus and because of the word of God” (Rev 20:4)

“They will be priests of God and of Christ and will reign with Him for a thousand years” (Rev 20:6).

“I saw no temple in it, for the Lord God the Almighty and the Lamb are its temple” (Rev 21:22).

“The glory of God has illumined it, and its lamp is the Lamb” (Rev 21:23).

“A river of the water of life, clear as crystal, coming from the throne of God and of the Lamb” (Rev 22:1)

“There will no longer be any curse; and the throne of God and of the Lamb will be in it” (Rev 22:3).

The above shows that God and His unique Son belong together. For example, they share one single throne (22:1, 3) and, together, they are the temple and the light of the New Jerusalem (21:22-23). Nevertheless, God is one Person and Jesus is somebody else. People who do not accept this VERY CLEAR conclusion will argue that Jesus is elsewhere called God. That is simply not true.

VERY FEW

The word theos appears about 1300 times in the New Testament. Of those, Trinitarians propose about 7 instances where Jesus is called God. The extremely small number of instances where Jesus is POSSIBLY called God shows that, in the other 99.5% of the instances, there is no dispute. In the other 99.5% instances, it is agreed that “God” refers to the Father ONLY. In other words, IT IS OVERWHELMINGLY CLEAR THAT THE TITLE "GOD" REFERS PREDOMINANTLY TO THE FATHER.

PAUL

Paul should be our main interpreter of the gospels. I have done a similar exercise as the above for the book of Colossians, which has Paul’s highest Christology, and I similarly found that the title God is used for the Father ONLY.

Furthermore, with the exception of two disputed passages in Paul's other letters, PAUL NEVER REFERS TO JESUS AS GOD.

The one is Romans 9 verse 5, but that depends entirely on punctuation. In 50% of the translations, the punctuation is such that it does not describe Jesus as God, but says that Jesus is blessed by God.

The other is Titus 2:13, which reads, “Our great God and Savior, Christ Jesus.” Trinitarians read this as referring to only one Person but it can just as well be a reference to two Persons; God and Jesus.

Just think of it: In all of Paul’s letters, which is about half of the New Testament, and which should be our main guide to doctrine, there are ONLY TWO instances where he POSSIBLY refers to Jesus as God. Since Paul never clearly refers to Jesus as God but maintains a clear and consistent distinction between God and Jesus (e.g., I Cor 8:6; 1 Tim 6:13), do we not have abundant evidence that Paul does not describe Jesus as God?

JOHN

The main verse Trinitarians use to say that Jesus is called God is John 1:1. I feel very passionate about this verse because the translators KNOW that theos is used in that verse in a qualitative sense. But they argue that this means that Jesus is like God in nature and that this means that He is God. Consequently, the average Christian reads the translation of John 1:1 as an identification of Jesus as God, rather than as a qualitative description. A better translation, I believe, would be something like: "And the word was with God and the word was like God."

The other verse in John is when Thomas sees Jesus after His resurrection and exclaimed, “My Lord and my God!” (John 20:28)! Can you imagine? Jesus never taught His disciples that He is God! In fact, in the very same chapter He refers to His Father as His God (John 20:17). And John summarises the purpose of his entire gospel a few verses later. Does he say his purpose is to show that Jesus is God? No! His purpose was to proclaim Jesus as Christ (John 20:31)! But Thomas, about 60 years before John made this summary of his gospel, miraculously simply knew that Jesus is God!

What the poor average churchgoer is not told is that there is a huge difference between the word theos and the word God. Hanson explains:

“The word theos or deus, for the first four centuries of the existence of Christianity had a wide variety of meanings. There were many different types and grades of deity in popular thought and religion and even in philosophical thought.” (link)

In Thomas’ day, the word theos was used for any immortal being with supernatural powers. And there were thought to be quite a number of such beings; including the Greek pantheon. In contrast, the word “God” is a name for one specific Being. Since the standard explanation of John 20:28 cannot be right, I think that Thomas used the word theos in a generic sense. Namely, seeing the risen Jesus, he used theos in the sense of an immortal being with supernatural powers.

HEBREWS 1:8

In Hebrews 1:8 Jesus is called theos but the very next verse refers to God as His theos. At least, that means that Jesus is subordinate to His Father. But Hebrews 1:8 is simply a quote from Psalm 45:6 where the king of Israel is called god IN A GENERIC SENSE. Hebrews 1 applies this to Jesus and also calls Him theos IN A GENERIC SENSE. It does not identify Jesus as God Almighty.

2 PETER 1:1

In the NASB, 2 Peter 1:1 reads “our God and Savior, Jesus Christ.” However, since this is the only possible instance when Peter describes Jesus as God and since, in the very next verse, Peter makes a distinction between God and Jesus (“The knowledge of God and of Jesus our Lord” (2 Peter 1:2)), we should read verse 1 as referring to two Persons; the Father and the Son.

CONCLUSION

The evidence that the New Testament refers to Jesus as God is negligible. Since the title “God” is consistently used to identify the Father in distinction from Jesus, when the angel told John, "worship God,” he referred to the Father ONLY.

I am not disputing that the Son always existed or that God created all things through Him. My point is that only the Father is the Ultimate Reality; the Source of all else, and that the Son is subordinate to the Father.

This answers the question: WHY did the angel not include Jesus? Why must only the Father be worshiped? Since only the Father is identified as “God,” all other beings, including His unique Son, are subordinate to Him. Therefore, we worship Him.

JESUS IS WORSHIPED.

But, as the question rightly states, angels and the entire creation worship Jesus (Heb 1:6; Rev 5:13-14). That does not contradict the statement that we must worship God only. The Greek word translated as “worship” (proskunuo) merely means to show extreme respect by falling down before somebody else. It is also used when people fall down before other people such as kings. For example, in Revelation 3:9, Jesus said, “I will make them come and bow down at your feet.” “Bow down,” here, translates proskuneó.

Furthermore, as we read in Philippians 2:9-11 and Hebrews 1:6, Jesus is worshiped by the entire creation BECAUSE THAT IS GOD’S WILL. There is, therefore, nothing wrong with showing extreme respect to the One through whom God created all things. But, as Philippians 2:11 adds, it is all “to the glory of God the Father.”