There are a ton of valid security, performance, and storage related reasons not to blindly support service workers on mobile. I'm sure they can be overcome, but I guarantee Apple won't be running to support SW until they figure out how to do so with the best result for users.

As an end user, I really don't want the result of following a link to a website (or 'web app' if you must, because app all the things) to be a progressive web app that a) consumes 100s of MB of my mobile device storage, b) runs in the background draining my battery and using my limited cell data plan, and c) send who know what information to god knows where. All because I clicked on a link and hit OK on an innocuous prompt...

Some of the capabilities presumed by service workers aren't even available to native apps on iOS because of these very same issues - why would Apple grant them to random websites?

Meanwhile they're pretty good at keeping up with simpler ways of blurring the lines between web sites and native apps, with APIs like AirPlay, Picture-in-Picture when you leave the browser, Force Touch...

I've always felt the WebKit/Safari team is just understaffed. IIRC there was graph posted here a while back showing the number of contributors on the major open source browser platforms and Google has a frightening amount of developers working on Chrome.