If you watch the admittedly long video from Tantacrul about what it took to get from 3.0 to 4.0 he talks about that. They had a lot of tech debt that made it impossible to update the audio engine. He talked about the new engine being the roadmap for 4.1.
I saw the Tantacrul video back when it was released. I think that's when I went ahead and bought myself a personal Reaper license. My thought then was: wtf are their priorities like if they'll focus on a complete overhaul of the admittedly basic but functional interface over broken recording and playback of audio?