I honestly think the biggest factor playing in shirt selection is making the highest number of people buy a new kit every year. If the the kit hardly changes between two seasons (including the sponsor on the front) there is very little reason to buy a new shirt, costing you £50, is there?
Moreover, this year we have a new partner with Adidas, which I was really happy about when I heard the news, but to be honest I've been quite disappointed with the new kit (I have to admit I was hoping for a return of the stripes). In any case, I think they will finally return in a year or two.
And same applies to the yellow alternative kit. There's an anniversary from that legendary victory in the FA Cup Final so it sounds like the perfect time for it.