From the website:
Credit expiration. – Credits do expire. You must use your credits to book travel before you receive your next set of credits. You do not have to travel before your credit expires, only book travel before the expiration date. You may travel up to 90 days in the future. If you do not use your credit before you receive your next batch of credits, the previous credits are forfeit.
I guess Alaska is hoping that a lot of credits are going to go unused. I'm not really sure who this is targeted at, maybe frequent Southwest fliers? Also the lack of Oakland service is going to hurt.