Slowly becoming less of a thing, but one of the major barriers to adoption thus far was the minimum iOS requirement of 13.

And not to mention the not-so-minor fact that Apple is promoting the use of async/await for many of the situations where we once would have used Rx or Combine.

Finally, like it or not, async/await is easier for some developer to get their head around and easier to teach than reactive streams filled with maps and flatMaps.