AP items cost

There was no real explanation why Riot Games made all AP items so expensive. Could someone tell me why? For example - Zhonyas - costs 500g more. In my opinion it's ridiculous. AD items are so strong now, Yasuo is killing everything on midlane and Riot NERFS AP champions so much. Why? My main lane is midlane and since 5.22 (to be honest, even for 1-2 months) and instead AP casters I see AD assassins and other champions there like Yasuo, almost in every game