The real question is "why are games like Omaha not no-limit?" - And the answer is that in Omaha and other circus games, equities run so close even on later streets that the correct play is almost always for whoever is ahead to bet everything and for whoever is behind to call. Playing it Pot Limit is necessary to turn it into a multi-street game of poker instead of a coin flip at the earliest possible opportunity. In a way, playing Omaha PL is like playing Hold'em (or anything else) Fixed Limit - if you're ahead, you bet as much as you can, i.e. as much as the limit allows, and if you're behind, you call because you have the odds to call because the limit was small enough to prevent the bettor from pricing you out of your draw.
Whereas in Hold'em, equities on made-hands-vs-draw scenarios are thick enough to allow for meaningful bets but thin enough that the correct bet size is usually a fraction of the pot rather than a multiple of the pot. That makes playing No Limit Hold'em a viable and interesting prospect; betting All In with a multiple of the pot is possible but it's reserved for specific situations, whereas in No Limit Omaha betting All In with a multiple of the pot would be a routine occurrence.