Because we've seen that a million and three times.
Problem with doing an Ultimate Cap book would be to find a point. The Ultimate books, when they're good at least, have a point behind them, a somewhat deeper message than, "Well, Marvel continuity is so unbelievably cluster****ed that we're just going to have our cake and eat it too by continuing to publish 616 books, but also just taking the easy way out and making up an entirely new universe." The Ultimates, for example, has a point. And it's good. Ultimate Spider-Man doesn't. And it sucks.
So what would be the REASON for an Ultimate Cap book? No, him being a dickbag, or him being stronger than 616 Cap, that's not enough. What's a good reason for it?