Why were both criminals that were crucified with Jesus abusing him in Mark, and not in Luke?

Luke was not an eyewitness, he just collect testimonies, as he stated in his gospel

For as much as many have taken in hand to set forth in order a declaration of those things which are most surely believed among us, Even as they delivered them unto us, which from the beginning were eyewitnesses, and ministers of the word; Luke 1:1,2

Mark is believed was the writter Peter used to write the gospel. Clearly Mark is more credible than Luke

Keep it simple!!!! The fact of the matter is in any multiple witness testimony is always going to differ slightly, because of our own individual take on things. In a court case it would be highly suspect if multiple witnesses gave identical testimonies, as opposed to similar testimonies. Both criminals, rebels, outlaws... Cursed him, but later one had a change in heart after seeing the way Jesus responded to His own crucifixion and mocking.

These two different accounts in Luke 23:39 on one side and Mark 15:32 and Mathew 27:44 on other side can be reconciled. by supposing that, at first, both of them reviled the Saviour, and that it is of this fact that Matthew and Mark speaks. Afterwards one of them relented, and became penitent-- perhaps from witnessing the patient sufferings of Christ. It is of this particularly that Luke speaks.

Or it may be, that what is true of one of the malefactors, is attributed by Matthew/Mark to both. The gospel writers, for the sake of brevity avoid particularizing, often attributing an instance to many, what was said or done by single persons. Meaning no more than that it was done by some one or more of them, without specifying the one.

Similar instance can be found by comparing:

Mark 7:17 When he had left the crowd and entered the house, His disciples questioned Him about the parable.

with

Matthew 15:15; Peter said to Him, "Explain the parable to us."

also

Mark 5:31, And His disciples said to Him, "You see the crowd pressing in on You, and You say, `Who touched Me?

with

Luke 8:45; And Jesus said, "Who is the one who touched Me?" And while they were all denying it, Peter said, "Master, the people are crowding and pressing in on You.

and

Luke 9:13, But He said to them, "You give them something to eat!" And they said, "We have no more than five loaves and two fish, unless perhaps we go and buy food for all these people.

with

John 6:8, One of His disciples, Andrew, Simon Peter's brother, said to Him, 6:9 - "There is a lad here who has five barley loaves and two fish, but what are these for so many people?

Source: Barns Notes

This potential discrepancy is addressed in Apologetics Press' question: Did Both Thieves Revile Christ?

Possible resolutions to the discrepancies between the accounts:

• Possibility #1: Initially, both thieves reviled Christ, but then one of them repented.

After hearing Jesus’ words on the cross, and seeing His forgiving attitude, the one thief may have been driven to acknowledge that Jesus was indeed the Messiah. How many times have we made a statement about someone or something, but then retracted the statement only a short while later after receiving more information?

• Possibility #2: The discrepancy could be due to a figure of speech called a synecdoche
  • Merriam-Webster definition: a figure of speech by which a part is put for the whole (as fifty sail for fifty ships), the whole for a part (as society for high society)…or the name of the material for the thing made (as boards for stage)
  • Note that the doctrine of plenary inspiration does allow for figures of speech, even as it applies to inerrancy.

Also, from the article in the portion relating to the possibility that this is a synecdoche:

It is feasible that Matthew and Mark were using the plural in place of the singular in their accounts of the thieves reviling Christ on the cross. Lest you think that such might be an isolated case, notice two other places in Scripture where the same form of synecdoche is used.

• Genesis 8:4 indicates that Noah’s ark rested “on the mountains of Ararat.” Question: Did the ark rest on one of the mountains of Ararat, or did it rest on all of them at the same time? Although the ark was a huge vessel, it obviously did not rest on the many mountains of Ararat; rather, it rested on one.

• In Genesis 21:7 Sarah asked, “Who would have said to Abraham that Sarah would nurse children? For I have borne him a son in his old age.” Anyone who knows much about the Bible will remember that Sarah had but one child. In certain contexts, however, one might use a synecdoche and speak of one child (as did Sarah) by using the word children.