Apple is dropping support for 32-bit apps in iOS 11. Doing this improves performance as 32-bit libraries need not be loaded alongside 64-bit ones. Furthermore, it paves the way for Apple SoCs that do not have hardware support for 32-bit instructions, maybe saving some precious silicon real estate.
They could just not load (or even ship) the 32bit libraries unless they were needed. My guess is they don't want to develop and maintain 32bit versions of all their frameworks, especially newer features like drag&drop. (For the hardware part I don't know enough to say whether or not keeping a 32bit armv7 mode around is costly?)
[1] https://9to5mac.com/2017/06/06/ios-11-32-bit-mac-app-store-6...