People stopped playing AP Champions for one reason and one reason alone: Assassins. Ignore what you are hearing about the tank meta, AP champions such as Morgana and Malzahar were very popular in the past, are strong currently and do % health damage that can melt tanks.
The issue with AP mids is the assassin meta. They are countered by the popular champions such as Zed, Leblanc, Talon, etc... With rise in popularity of assassins/high mobility champions such as Yasuo going mid these days, if you play a classic AP mid you will get counter-picked and have a rough time.
Note that the few AP champions, such as Ahri, that are popular are capable of huge bursts themselves. Ahri is also very mobile.