This is a good question--and believe me, I've considered it at times.
The price of REAPER actually isn't unprofitable.. Provided you get enough people using/buying it, which we are.
A big factor however, is that if REAPER was open source, it would be much more difficult to support proprietary standards.
The licenses for VST and ASIO are incompatible with GPL'd software, Propellerheads will only license ReWire to non-open-source developers, etc.
Get Steinberg and Propellerheads to open their stuff up, and we'll think about it
-Justin