Could they write?
The first, and perhaps most obvious answer may very well be that they didn't know how to write. Recall that most of the disciples were—by and large—peasants. Even the tradition that Peter was the source behind Mark doesn't affirm that Peter wrote the gospel, only that someone else (named Mark) wrote it down according to Peter's "memoirs" (or whatever; I don't really buy this theory, personally)
Did they need to?
A second reason is linked to early Christian eschatology(-ies). It seems to have been commonly believed (perhaps because Jesus said it: Mk 13:30, Mt 24:34, Lk 21:32) that Jesus would be returning within the lifetime of his earliest followers. It is supposed that this concern is what is at the root of 1 Thess 4:13-17:
But we do not want you to be uninformed, brothers and sisters, about those who have died, so that you may not grieve as others do who have no hope. For since we believe that Jesus died and rose again, even so, through Jesus, God will bring with him those who have died.* For this we declare to you by the word of the Lord, that we who are alive, who are left until the coming of the Lord, will by no means precede those who have died. For the Lord himself, with a cry of command, with the archangel’s call and with the sound of God’s trumpet, will descend from heaven, and the dead in Christ will rise first. Then we who are alive, who are left, will be caught up in the clouds together with them to meet the Lord in the air; and so we will be with the Lord for ever.
People were dying and Jesus had yet to come back. So the question is, if these early Christians believed in the immediate return of Jesus, why would they write a book about him?
On the other hand, the Gospels generally hail from the end of the 1st century CE (though Mark, perhaps 67 CE and John perhaps as late as the 2nd century CE). In other words, when these Christian communities were coming to realize that it would be important for them to record and pass down their faith tradition to the next generation
Yes there are many other "gospels" that were written about Jesus. Sometime if you really want an eye-opener you should find a copy of The New Testament Apocrypha. It's a two-volume set edited by Hennecke and Schneemelcher. Lots of good stuff in there, but most all of it is pretty late.
The only "controversial" non-canonical "gospels" are Q (the putative source behind the material common to Matthew and Luke not found in Mark) and the Gospel of Thomas. However, neither of these are "gospels" in any meaningful sense, since they are both just collections of sayings. Q is only controversial in the sense that a growing number of scholars have become suspicious of its existence, and Thomas, because some scholars want to claim that it's just as early as the canonical gospels. I'm not convinced, but I'll let you decide for yourself.