So why didn't they write about the Ascension in their letters? The letters long pre-date the Gospels. We have to rely on the hearsay of Luke dozens of years after the fact, and no eyewitness account. If, in your estimation, proximity is truth, your logic makes Luke's Gospel especially specious.
Luke knew both Paul and Peter.
"So why didn't they write about the Ascension in their letters?"
Paul did write about the ascension. In fact he saw the risen Lord in person on the road to Damascus.
Eph. 1:19-21, "And what is the exceeding greatness of his power to us-ward who believe, according to the working of his mighty power, Which he wrought in Christ, when he raised him from the dead, and set him at his own right hand in the heavenly places, Far above all principality, and power, and might, and dominion, and every name that is named, not only in this world, but also in that which is to come:"
Eph. 4:8-10, "Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men.
9 (Now that he ascended, what is it but that he also descended first into the lower parts of the earth?
10 He that descended is the same also that ascended up far above all heavens, that he might fill all things.)"