Because you have to pay developers to maintain it. Developers are expensive. At some point it doesn't make sense to keep doing that, so products are end of lifed.

You're more than welcome to attempt to flash a custom firmware on it, though. I'm sure there are devs working on it.

Also, that 20 year old computer is running a general purpose OS that is designed to work on just about any system. The OS on a smart device, especially one from 2014, is heavily customized