I think, largely, the same reasons apply to Linux vs. BSD in supercomputers as Linux vs. BSD generally. You might as well ask why Linux and not *BSD is used in Android, on servers generally, or by large technical knowledgeable organizations such as Google, Amazon, Facebook, etc.<p>So, in no particular order:<p>- Linux came on the scene when BSD's were mired in legal uncertainty. After the legal issues were settled, Linux had already become the default choice for someone wanting a FOSS Unix-style kernel, and the BSD's never caught up.<p>- The GPL license meant that improvements were shared rather than squirreled away in various proprietary spin-offs and thus lost when whatever company was behind them folded (generally, exceptions going both ways surely exist!).<p>- Due to Linux gaining the initial momentum, developers flocked (and keep flocking!) to it, leaving the BSD's ever further behind.<p>- Linux was more welcoming to new contributors, whereas the BSD's were controlled by a small circle of core developers sitting on the commit access. And of course, the BSD way of solving disagreements was forking the entire thing, further splitting up the already small developer base.