I'm sure we've talked about this before head [busmile]
The process used to collect NT books together was rigorous, consistent and agreed by the great majority of believers. Basically, the conditions books had to fulfil to get in the canon were;
-the books were widely used by the early church, in agreement with each other and with the other books used
-the books were all recognised as having some kind of apostolic origin or at least connection
There were many early Christian writings/letters which were in line with beliefs and widely used, but they were left out as canon and left as 'helpful writings', obviously they were not viewed in the same light as the 27 books which were included - probably for the second reason above.
As for the 'rogue gospels', eg Thomas, Mary etc, they were left out simply because they were a/not widely used and b/not in any kind of agreement with the vast majority of writings - canonical and non - that were used. Some of the premises in these 'gospels' were ridiculously out of step with the rest of the information available. Don't forget the rigorous oral tradion as well - there would be a lot of very detailed information handed down from generation to generation, and the 'lost gospels' were out of step with all this.
It's quite simple - these writings were not included because they were not widely used and agreed. It would be like including some obscure self published badly written kindle book with a great literature anthology - simply not comparable.