serious answer: because mobile phones actually do have (in certain specific scenarios) "better" security than desktops, and secondly as a calculated strategic/political move to maximize reach/impact
mobile phones tend to have actual security boundaries between apps, whereas desktops traditionally do not and only have security boundaries between users (which isn't very useful on single-user *personal* computers, as this security model predates them)
like it or not, the balance between desktop/mobile traffic on the internet has completely flipped over towards mobile about a decade ago, so "most users" really are accessing the internet via mobile devices. part of making "privacy" software actually useful is that "normal" people also need to be using it (you don't want to become "the app only used by drug dealers and terrorists")
it's a shitty situation without any easy ways out