Why? I've been in a lot of studios and never seen Reaper used once. It's all Pro Tools and a surprising amount of Cubase. This is very much the same as "if you want to get serious about coding use VIM not VSCode".
Because its extensibility is enormous and it has a thriving community of people developing for it. The JSFX alone has made it possible to replicate thousands of dollars worth of dsp algorithms for everyone for free and The developers listen to concerns of the users. For me it's a DAW that represents longevity in a creative practice.
this is highly dependent on region, genre and demographics. every scene has their own tools. in the EDM scene, Pro Tools is regarded as a joke -- absolutely no one uses it, and the impression is that it's for old and out-of-touch people. I know a team producing music for several very well-known television series entirely using Reaper
In the BBC remote vans they run REAPER alongside some other proprietary stuff.