Because that would draw people away from flying AS to LAX and then AA to NRT. If AS launched SEA-NRT, they'd probably do well (this is one of UA's most profitable routes), but at the expense of feeding OneWorld traffic to AA's LAX-NRT flight. I don't know how well AA does on their own from LAX (they have tons of competition from UA, Star, and OneWorld), so this might be enough to make the flight unprofitable for AA and forcing them to abandon it. Mind you, this is pure speculation and maybe LAX-NRT does fine on it's own, so it would just be AS vs. UA.

As to the 739X, it's just what the 739 should have been in the first place (IMNVDHO). So a fleet of 73G/738/739X (so AS converts any future 739 orders to 739X and swaps their existing 739s to 739Xs) would give AS the ability to serve all their current markets with just the right amount of capacity. With a seriously fancy flight management system, you could darn near do it in real-time, meaning every flight goes out with just the right amount of folks - "Just in Time Seating".

And yes, all 737NG equipment have common pilot ratings and I imagine the flight attendants do not need any special training across the models, either. And maintenance should be identical across the board, including parts and they all use the same model of CFM powerplant (just with different thrust ratings).