Mac Generation suspects that the decision might have something to do with the Widevine copy protection plugin. Spotify wants to use that for web-based music streaming, but macOS may flag it over concerns that its security isn't sufficiently airtight. And without reliable access to Widevine, Spotify can't guarantee that Safari will play properly even if it's technically possible.

This isn't likely to be a huge issue on the Mac, since it doesn't take much to fire up another app. However, this could be a headache if you use a Mac at work and aren't allowed to install your own software. Also, it illustrates one of the perils of copy protection on the web: it turns an ostensibly universal platform into a proprietary one where you have to support a given company's add-ons to get the full experience.