My brief take on why they didn't work is because of timing. If you're trying to revive something, you'd better make sure you're reviving something that people actually miss. It had only been a few years since the Timely heroes had gone away; they had had their 40s heydey and needed to just go away. The 50s revival was pointless; nobody felt any nostalgia about those characters; it had been too recent. Their original readership was in college by that time and too old for comics. Younger kids had moved on to other genres.

By the early 60s, when the heroes came back...it meant something. By then Cap was a piece of nostalgia...he'd been gone technically for almost 15 years. Dads who'd read him in the 40s were buying it for their sons. There was a lot of nostalgia--he'd been missed greatly, and needed in the 60s as a symbol of America. Remember in Avengers #4 when the cop saw Cap return and broke into tears? It was a funny scene, but also very poignant...it meant something. That's all about it, right there in those two panels. The 50s revival meant nothing; the 60s revival meant something. That's all.