For DC, either Julius Schwartz never gets into the biz, (thus no real Silver Age) and sticks to sci-fi magazines, OR Siegel and Shuster's cases against them over the rights of Superman manage to completely bog the company down.
Marvel's pretty easy really... Stan Lee quits Marvel (which he WAS considering) before he can bring in the Fantastic Four, the Avengers, Hulk, Spider-Man, etc. Marvel tries to compete, but finds itself gone within a decade.