Why did Jesus call Abel a prophet?

It's not necessarily the case that the Evangelist intended to depict Jesus as implying that both Abel and Zechariah were prophets. Jesus has been talking about the killing of "prophets and apostles" (verse 49). He may simply be using the deaths of Abel and Zechariah as "brackets". The New American Bible, Revised Edition, offers a note to the Gospel passage:

From the blood of Abel to the blood of Zechariah: the murder of Abel is the first murder recounted in the Old Testament (Gn 4:8). The Zechariah mentioned here may be the Zechariah whose murder is recounted in 2 Chr 24:20–22, the last murder presented in the Hebrew canon of the Old Testament.

In other words, this translation suggests, by "the blood of all the prophets shed... from the blood of Abel to the blood of Zechariah" Jesus may have meant something like "the blood of all the prophets murdered in the history of murder."

This is certainly not the only possible answer, but it is the only answer I've seen that doesn't require an investigation or interpretation of the words and acts of Abel.

The broadest sense of an OT prophet was the Spirit of God coming up or moving a person to reveal something about God. The narrow sense is a prophet speaking "thus sayeth the LORD". It is in the former sense that Christ calls Abel a prophet.

And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD. And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering: ... If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him. Gen 4:3-4,7

Prophet: "one who speaks forth or openly" (see PROPHECY, A), "a proclaimer of a divine message," Vines

So, how was Abel a prophet? What was the divine message from God as regards the offerings of Cain and Abel?

Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them. Gen 3:21

Obviously from the very first example of God slaying the animal to cover the sin of Adam and Eve, the prophetic voice of Abel is one of blood sacrifice that culminates in Christ Jesus.

Why did Jesus call Abel a prophet?

Jesus did so because Abel was a prophet. Not all prophets have a recorded prophecy to pass on to future generations. Our Lord is simply identifying Abel as the first martyr and prophet in the Old Testament and Zechariah as the last Old Testament martyr and prophet.

Abel performed a prophetic act by bringing a lamb to offer to God centuries before God gave the Law that required it. Abel knew the promise to his parents, of a Savior coming, would require that His sinless God would have to come as a Lamb, long before the prophets who prophesied it more clearly with words and metaphors. Abel was a forerunner as were all the saints before the Law was given, who understood sacrifice before it was ever law. Abel was the first recorded to offer a lamb, and by it he prophesied the coming mission of Jesus Messiah. Many future prophets would perform prophetic acts in keeping with the plans of God, such as one described in Ezekiel 4 by that prophet, foretelling both soon and far-future events and missions in God's strategies for Israel and mankind. Or Isaiah 20:3 when God refers to his walking naked and barefoot three years for a sign and a wonder. It is a sign and a wonder to me that Abel knew so clearly, that the prophesy of a Lamb, a firstling of the flock, was the perfect and only suitable offering. That it was the perfect living metaphor that would be used for all the thousands of years thereafter as the picture of Jesus. As a prophet, he understood what the future held and he was looking forward to the Promise. And His blood still cries out the injustice of his premature death by the murder of a brother jealous over the favor upon Abel for his insight, that could only have come from an intimacy with God that exceeded Cains. And that pattern continues in the Church and religion today: the jealous and the presumptuous persecuting those who are evidencing the favor of God in countless and glorious ways. - Why is Abel called a “Prophet”?

In Hebrew, Christian, and Arabic traditions and legends it is said that God showed his acceptance of Abel's sacrifice by sending fire to consume it, as in 1 Kings 18:38.