Since OP is flying LHR - LAX, the aircraft is 777-300ER

Thus the MCE in this aircraft is 3-3-3 against 3-4-3 in coach, plus more legroom in general.

So yes there is a premium attached to this seats due to more legroom and more width.

Yes these seats have a price tag for non-elite if you are not able to choose upon check-in.

OP should have asked when check-in and the ground staff sometimes can give the real reason if both (OP and staff) are friendly.

Unfortunately I would not be surprised when AA copy what DL is doing for its MCE version.

Next year all these MCE seats on B77W will be replaced by Premier Economy and the new MCE section will be as same as the Y seats in term of layout and width.

SO I say it is tough luck, but OP should press further to find out why he lost the seat assignment. But do expect this to happen more in near future before Premier Economy finally role out, or AA announces the same MCE procedure as Delta.