Main thing for me, besides the clean, simple interface with Studio One, is the feature that allows you to set your buffers to the lowest setting and then playback is run as if it is higher, while allowing you to record new synths without the additional latency. I never remember what it's called because it's one of those things that I turned on when they implemented it, and I experimented briefly to see how it works and set it the way I wanted it, and rarely touch it any more. Basically it allows you to get into mix mode without having to turn up your buffers as you start getting a heavy mix with lots of plugins, many of which add their own additional latency due to adding buffers to the chain, and if you to add a new synth track or want to monitor a vocal track with effects, the track you enable for that is still playing at minimum latency instead of lagging due to the buffers and plugin latency on other tracks that are synced with it. It's basically like two sync engines in one. Playback is synced to 512 or 1024 and the live monitoring is set at 32 or 64. Makes a big difference for my workflow. Plus it is stable.
And I agree with you that SONAR is slow, clunky, and sometimes a bit unstable. Rarely get crashes, but I think they have been using old code-base and adding bloat for way too long. The only reason I still occasionally use it because I learned on it and have some old projects that I occasionally like to revisit and redo a few tracks with new equipment to see how it sounds. And even then I'll usually more likely to export the stems or tracks and save the MIDI file so that I can work with it Studio One, as I just like it better.
Reaper doe a LOT with a very small setup program, and for that it is extremely efficient. In many ways I rank it right up there with Studio One, and if it weren't for that feature I mention above, I'd probably work harder to get it set up to my liking and learn it better. Really can't knock it for trying to be everything for everyone, but it has so many customization options it can slow me down sometimes trying to dig in and figure out how to do something. And I really don't know how they can make it that flexible and powerful without it being bloatware. Pretty amazing program, and I will most likely shell out the cash to get the next version just to see how it develops, although I think my license will get me up to any Version 6.
I haven't tried Pro Tools in a long, long time, but it's basically what
@Stratman68 says. It was the early king and had a head start. It has long been surpassed by almost everything else out there, but those studios that invested in it early, and the seasoned "pros" that don't want to learn new tools are what keep it going at all.