: I guess most people (me included) doesn't like to change their minds on something they thought they knew, they don't want to admit they're wrong. No matter if it was a mistake or not, the Silmarillion says that Gil-Galad was the son of Fingon so when you read the PoME many years later, it is hard to believe that Silm was wrong, so a lot of people denie it. I also believe that Gil-Galad was the son of Fingon, but I know I can't prove it.

: Maglor

JRRT had several conceptions of Gil-Galad's lineage. At one point,(IIRC while initially writing LotR), Gil-Galad was a descendant of Feanor (this, obviously, did not make it into the finished product). JRRT also considered to have him as the son of Finrod (and brother of Galadriel!). There was also the conception of Gil-Galad as the son of Fingon (and CT implies that no change from this concept was made on the family tree the professor did). JRRT last conception seems to have been that Gil-Galad was the scion of Orodreth (however this was only his last conception, and that does not mean he would not have changed his mind yet again, and made Gil-Galad someone elses son, if he had published an authoritative geneology).