It's not so much that I "left" Reaper, as I still use it all the time. But I've given up using it for most things that I found it frustrating for, and instead of banging my head against the wall use other tools that work no muss or fuss. Otherwise, it gets loads of use here for what it excels at.
Midi work where the project isn't an EDM or otherwise looped and quantized project, as in non-quantized (and even not played to a click) tracks I just do elsewhere. IOW 95% of my time if in midi. The simplest midi editing is arduous in Reaper, and the fact that there are 4,000 user scripts to do normal and wild things that aren't otherwise natively available doesn't help that there are so many gaps when needing meat-and-potatoes and not esoteric functions. That Reaper is item based/reliant and not track based means that you...oh never mind
Also, projects with many tracks that need easily create-able, editable and accessible groupings and takes. Yes, Reaper can do those but my life went back to normal when I stopped being frustrated with how, yes, it's a wonderful audio program for $60 and I remain a fan, but it really is NOT so configurable that it does everything any other DAW does and better. Sorry, no. And if you look up "Putting lipstick on a pig" in the dictionary of slang there's a picture of Reaper with 50 themes.
If you have a reasonably challenging project it really is the Linux of DAWs, which is great for some and a dealkiller for others.