The whole point of John's Gospel is to show its readers (and listeners) that Jesus is Christ, the Son of God (20:31), which goes back to Psalm 2 where God installs David to sonship i.e kingship, where David "ascends" to the throne, as it were, as a type of Christ.
The means by which John aspires to prove Jesus' Christhood, are signs. Some pointing back to the Prophets, and some, forward toward The New Era. As the ascension of Christ would be one of the most important, if not the most important sign to prove that Jesus is the Christ, John seems on the surface to fail to drive home his point by omitting that from his story, as it is displayed in the Synoptics. However, going back to the first clause above, it would seem to be circular reasoning, superfluous and contra-productive, even to try to prove an event with a sign that is that very event, and which you're actually trying to prove by presenting other signs.
If Jesus' identity as the Messiah has come into question because it can't be proven he actually has ascended up into heaven, and the Messiahship is connected to that ascension, it wouldn't be helpful to impart the occasion into the story. However, the ascension is mentioned in 20:17:
"Jesus said to her, “Do not hold on to me yet (still), for I have risen [from the grave] toward my Father to yet (first) go to my brothers, to say to them that I am ascending to my Father and your Father, my God and your God”
Then the words in verse 21 are characterized by a clear "farewell"-language. Only the event itself is missing. And the content of the remaining 32 days after his last encounter with Tomas, a week after, is by Luke not displayed anyway.