So many reasons. One is definately fear; fear of interfering with a divinely inspired canon. Whether you believe the Bible is infallable or not you have to ask the question of whether the decisions of the Councils in the 4th Century were infallable seperately. Leaving that aside though, we are in a far worse situation in terms of determining canon 1600 years later because we are fairly certain that the Church fathers had access to better records and more complete information. Additionally a lot of these "new" Gospels were only lost prior to the reformation and at least some were known in Mediaeval Europe and Byzantium, at that time the "Bible" or "Scripture" was much more a body of writings than a "book" in the modern sense, so that non-canonical texts were less distinguishable.

Ultimately the canonical texts are those considered to be of reliable providence, and modern scholarship hasn't really challanged this in a serious way. No earlier Gospel has been claimed, for example, and while Thomas and Peter may have a great deal in them which is true they are a century or more later than John, which is the latest of the canonical texts, and the most divergent.