The entire point was that, for all the maturing Tony did in the first film, swearing he wouldn't make weapons any more… that's what he was still doing. the second movie made a huge deal about him creating weapons that would get out of control and had a government hearing over it where he swore up and down "these weapons won't ever be a problem because it's only me using them..." though of course he got drunk, and Rhodey (and thus the government got a suit) and another villain got access to his tech and made a small army... , and in the third film he built a personal army that ended up only being on even ground with different threat entirely... and this was after the alien invasion in Avengers..
Despite what he's doing though, there's still other companies and individuals stealing his tech, mad scientist creating monsters, asguardian gods, and aliens also attacking. Tony being his old self, being reliant on the armors... is just escapism, not dealing with things that his genius could be better suited for. (And he spent most of the third movie OUT of the armor.) Even making all that stuff, he still couldn't protect his home, had trouble saving the airline passengers, still couldn't beat the badguy, still couldn't save Pepper, and in general, the months he'd spent making 40 different suits... while allowing him to survive that fight, did him no good while other guys made even more powerful weapons without him, or because of him. For all his cleverness, all his preperation, all his promises and wit, the woman he loved still died because of his mistakes. (That Pepper survived thanks to the Extremis wasn't due to him at all, and he had no way of knowing at the time that would happen.)
In Avengers, Cap asked Tony what he was without the suit, and Tony gave a glib reply about "billionaire genuis philanthropist playboy"... but it still didn't answer the question... is he a hero without the suit? (And then he nearly sacrificed himself after, so the answer is "yes".)
He's moved on now, partly because its what Pepper wants him to do, partly because even having like 40 suits at his disposal didn't help out much against other threats. Thats him hiding from his problems, his responsibilities, his life. Now, he can be bigger and better, and move onto new things. Back to focusing on what's important in his life. Greater works, Pepper, making the world a better place, not just making bigger and better guns... Tony can focus on super soldier serum, or cures for cancer, or helping out starving countries... being a better man... rather than just a guy that makes personal guns... which is what he was 10 years ago, what he swore he was trying to get away from... but wasn't.
But of course he's gonna have at least a suit in Avengers 2.
(also, it's implied that at the end of the film, Pepper's extremis powers were removed... while Tony had them added to himself to help fix his heart problem.)