I'm not sure, not having any experience of proprietary OS development. I could guess. The technology was not an insignificant thing (RCU as mentioned). Possibl

I'm not sure, not having any experience of proprietary OS development. I could guess. The technology was not an insignificant thing (RCU as mentioned). Possibly more important was that Linux didn't have such a pressing need to scale up, whereas for Solaris they were a hardware company that made these big systems so they had to get it working at all costs. Linux's main market was 1 CPU systems for a long time, and there was strong push back against complexity or slowdowns caused by scalability improvements so the bar for them was very high.