The reason given at the time was the desire to simplify maintenance and parts logistics.
Now Alaska's fleet is all 737, a mix of MAX and NG models. The MAX is about a quarter of their fleet, so the current grounding has hurt them.
How would this help them? A large % of the fleet is still grounded.
As Alaska is based in Seattle, they've been pushing this ridiculous all-Boeing nonsense for a while. Horizon still exists as a wholly owned subsidiary of Alaska. Their fleet is all Embraer. In fact I'd be hard pressed to think of a time when Alaska was ever all Boeing.
I do kind of wonder about the relationship with Boeing, with Boeing Field and the plant just up the road from Alaska's HQ at Sea-Tac. Cynically speaking I'd guess that Alaska and Boeing CEOs are golf buddies or something. Boeing's HQ is is now in Chicago (which may be part of the problem, as the article notes) so maybe not.