Because, if you want to use one of the other 2 cards in your hand to combine with the 3 community cards, than only one of your a's is valid meaning everyone else is entitled to the other pairs that show up and will probably 3kind you out.
Now you are offering Omaha wisdom? Have you even played more than a dozen hands of
real money Omaha?
AAxx is a strong hand in Omaha. If the money goes in before the flop, you will almost certainly have the best of it. But, the game is pot-limit and the money almost certainly won't be all-in before the flop. You are probably going to see a flop with it.
In hold'em hand values are fairly static. If you had the best hand preflop, you will have the best hand on the flop the vast majority of the time. And the best hand on the flop remains the best and the turn most of the time. Same thing for turn to river.
This is not the case for Omaha. Hand values are very dynamic. AAxx will still be the best hand (heads up) on a lot of flops, but significantly less flops than AA would still be best in hold'em. And, those times when it is best, your opponent could have multiple draws, pair plus draws, and so on to where they have the
equity to get the money in and see the river. If you 3-bet your AAxx hand (which you should most of the time), it is usually obvious what you have. Most players don't 3-bet other hands often enough.
It gets worse. You don't know what draws your opponent has on the flop. So a board of 9c8d3c is just horrible when you have AAxx and no clubs. Any club or a 5, 6, 7, 10, J, or Q completes an obvious draw your opponent might have. If the 8 or 9 pairs, they could have trips. That is like 2/3rds of the deck that could have changed the leader. On the turn, if there is still lots of money, you will have to guess if your opponent is
bluffing or hit.
The nuts on the flop is rarely the nuts by the river. AAxx is still a powerhouse hand and should be treated as such. It just isn't as obviously profitable as AA in hold'em.