Mixer is a platform with a terrible name and a bunch of issues that it shouldn't have. You should have working clips, VODs, and cool features you can't get on Twitch. Instead it has none of that and it looks worse. It's a worse Twitch.
It has both clips and VODs (though I think Clips is in beta and currently partner only until it exits beta). As for cool features it has almost no delay (if using the right software), though Twitch has caught up somewhat (Mixer is still faster in my experience), Mixer also allows streamers to get paid via viewer interactions without having to reach certain milestones like on Twitch or Youtube. There's also actual viewer interaction (which sadly mostly gets used for soundboards or chat commands but can be used for much more).
I understand why Mixer isn't doing as well as the other three (Twitch was "first" and went for ages with no real competition, Youtube and Facebook had an established brand and userbase before jumping into streaming), but let's not act like Mixer isn't as feature rich as the other three.
Microsoft hasn't really been taking advantage of the fact that they have and make games. They demoed viewers being able to impact a streamers Minecraft games, but besides a handful of games like the Darwin Project taking advantage of it, we haven't really being seeing any first party's do anything with audience interaction, probably as a result of Microsoft's hands-off style with first party studios.
Still, imagine if you had a Mixer mode in Gears 5 which allowed viewers to switch your weapons for Sparks (Mixer's free currency) or have the audience dictate what season it was in Forza Horizon 4. There's a lot that could be done with this feature that's not really being done. And I'm sure the developers at XGS could think of much better ideas.
I do think that a major reason its so hard for viewers to adopt Mixer, besides the fact that they have to go to a different site/app, is the fact that they don't have a native app on all the platforms that Twitch or even Youtube do. Now, why is there no Mixer app on PS4? Seems like it'd be logical for Microsoft to want Mixer on all platforms where gamers are.