[saagarjha]

Catalina runs on all Macs that support Mojave, which I assume influenced the decision. (I didn't see an iOS 13 update, which helps bolster this theory.)

[why_only_15]

My guess is that iOS 13 didn't drop support for any devices, and Apple is only releasing a patch for devices that can't upgrade to the newest OS.

[saagarjha]

This is also my guess ;)