The man behind Reaper (Justin Frankel) is a genius when it comes to software/programming.
No different than a musical prodigy...
Take that skill... and start with a clean slate.
Take some of the best bits from all the top DAWs (especially early on) and put an improved spin on them.
Reaper started as a "spin-off" of Vegas Pro (Justin was frustrated with lack of audio specific features/development when Sony bought it and shifted focus to Video).
"Necessity is the mother of invention"
Small dynamic company that can incorporate changes/improvements quickly.
Their situation business-wise is very unique. They're not reliant on Reaper turning X amount of profit.