Earlier quoted context omitted.
Rumor has it that, despite Apple's public claims, one reason why is that iMessage is tied to a physical device. This helps keeps spam, malware, and banned devices all under control. Expanding to Windows and Android by definition means that iMessage can no longer have that per-device tie, which undermines these controls.
This is a total bullshit reason. Literally every other messaging app, including well regarded secure ones, have cross platform versions available.
All while I receive whatsapp/telegram message spam on a fairly regular basis, despite not being active in any groups or channels on those apps (and in general, opening them maybe once or twice a week at most). The only reason I even use those 2 apps is to talk to select few old relatives of mine who are not very tech literate and who just happened to learn how to use those apps +a couple of friends who simply insist on using those apps.