All the books that were excluded were under much debate. They were either included or excluded from various canons, with the exception of the Didache, from I can remember. That is more of a book on Christian living and was never considered Scripture, again, from what I can remember.
This process of canonization and which books to include or exclude went on for well over a hundred years, possibly just over two hundred, depending on when the "Muratorian canon" is dated--
http://en.wikipedia.org/wiki/Muratorian_fragment.
This was not something taken lightly, not that you're suggesting that. As one can see from the Muratorian canon there were many books that were already widely accepted and that are in our current NT, ones that were never really in dispute at all.
As to why some were excluded, it is likely that they didn't meet the criteria Nathan made in his first point and were perhaps not as widely known as the others.