Like many of Marvel's 70 TV efforts, Captain America didn't really hit the mark.
If you want a singular reason as to why essentially none of the Marvel Comics characters who were given a shot at live-action TV series worked out other than The Incredible Hulk -- and, in case you forgot, that list has thus far included Spider-Man (twice!), Doctor Strange, Thor and Daredevil -- it's probably this: Superheroes (with apologies to fans who're still having an existential crisis over this fact) are fundamentally characters created for children.
And while stripping the Hulk to his bare essentials worked in terms of turning him into something that could work as an older-skewing series, making costumed do-gooders like Spider-Man fly with grownup audiences just wasn't going to happen. I mean, sure, a mostly-faithful adaptation had worked for Wonder Woman in the same era, but if we're being honest the adult popularity of Lynda Carter's series may have had more to do with what her costume wasn't (read: modest) than what it was.
It's not surprising at all, then, that the sole non-failure of the bunch was the Japanese Supaidaaman. Sure, his backstory and rogues gallery were completely reworked, but by the fairly low bar set by every western superhero series of the era (remember: the Adam West Batman had been off the air for over a decade by this point), well... it was Spider-Man beating up a bunch of colorful bad guys for 22 minutes an episode. See, while kiddie TV of the 70s in the U.S. was operating under mutually-agreed guidelines that children's shows be largely non-violent and focus on prosocial character-building, Japanese kid's TV was living in the exact space as U.S. comics were: Rigidly-formulaic tales of costumed people punching each other -- a match made in heaven.
Ironically, Captain America -- apart from his name, obviously -- was probably the only guy in the Marvel stable closer to the Japanese model of a children's TV superhero: Brightly-colored costume whose design informed his theme and attitude, cool signature weapon, looked "right" striking authoritative poses while dispensing lessons and wisdom. Toei (who had the deal with Marvel that resulted in Supaidaaman) thought so, and planned a revival of the then-moribund Sentai franchise that would've teamed Captain America with a newly-created Captain Japan and Captains of other nations... a concept they decided was so strong it ultimately didn't need Cap! The result, Battle Fever J instead featured "Ms. America" (!) on a nation-themed Sentai team who were also the first of their kind to pilot robots (a conceit borrowed from Supaidaaman). The show was a smash, reviving the Sentai franchise and thus becoming the direct progenitor of Power Rangers.
The point of that digression? Japan knew that Captain America would be great as the lead of broad, action-packed kid's TV superhero show... but America didn't.
To be fair, the two Captain America TV movies made (once again) to serve as pilots for a series that never materialized are easily the most "comic-booky" of the 70s Marvel cycle (discounting the Thor/Daredevil stuff in the Hulk movies, which came later): He has a costume, he has powers, he's got gadgets and gear and he battles what more or less qualify as supervillains -- he just doesn't do any of it well. And the tone seems to be there less because someone among the producers had actually figured out how to let superheroes be superheroes than because they'd decided that if "borrowing" the outline of The Fugitive had worked for The Hulk, Captain America could be translated to TV by "borrowing" from The Six Million Dollar Man.