Most likely because it is a pain to maintain the 32-bit APIs (applications programming interfaces) and the 64-bit APIs, especially when you have to make sure you are patching security holes, and making sure the kernel side does the right then when getting a 32-bit address. (I've had to do this in other operating systems, it is a lot of work).
And it is not just the 32-bit kernel APIs, it is all the 32-bit frameworks, which are very extensive.
Apple is maintaining macOS, iOS, watchOS, tvOS, whatever the HomePod OS is called. All of which are similar, but different.
There are a lot of lines of code to maintain (like millions), and simplifying the effort just improves the development process all around.
Yes it is a pain when Apple makes this kind of switch, just like when they switched from 68000 Motorola CPUs to PowerPC, and then to intel CPUs. After the switch there was a transition period, but eventually the old apps stopped being supported. The same is true for 32-bit apps, there was about a 10 year overlap, that is not a trivial period of dual support.