I think that, because he was the first hero to appear, it took some time for other heroes to appear. To Zero, saving the people he cared about and the world, was more important than remembering allies who popped up to do the same thing.
Besides, Zero was so strong on his own, that he basically didn't need any help when fighting.
You can also assume that because he was so chuunibyou that he just couldn't care less about other people, only saving the world with his power. Worrying as it may be.
I get that part, im just surprised he didnt even remember their faces. like some vague "hmmm, these guys look familiar...oh yea!" at least.
Maybe it's because before heroes usually wore masks, so he won't remember their faces. And I suppose most people saw him from a distance, so he didn't have any chance to 'socialize' ┑(￣Д ￣)┍ I don't care he's cool either way.
I'm a bit concerned at how our MC (the "real" zero) has practically no recollection of any of these previous allies of his. Shouldn't he know all the SS ranked heroes pretty well? How freakin high was he during his glory days that he cant remember any of his "allies"?