It's because SMS is unencrypted, and this is an effort to have secure communication between iOS users and Android users (since iMessage is Apple-device-only). There's also a status factor, where Android users are lowly "green bubbles" in iMessage, which signals decreased security and questionable support for the interactions available to iMessage users.
Of course everyone can switch to Signal and this is a moot point, but millions of Apple users use iMessage and aren't likely to switch.
Of course everyone can switch to Signal and this is a moot point, but millions of Apple users use iMessage and aren't likely to switch.