Reading things like http://blog.erratasec.com/2013/02/custom-stack-it-goes-to-11... suggests that it's probably simply the case that the OS itself is sort of a side-issue once you need performance since you're going to be bypassing the normal network stack anyway.
At that point ease of management or package installation probably matters more to developers and it might simply come down to things like driver support and other stability/performance issues where Linux has gotten a LOT of highly-specialized attention from hardware vendors and the HPC world. Back when 10G hardware was just starting to enter the market, we bought cards which came with a Linux driver but it took awhile before that was ported to FreeBSD and longer still before the latency had been optimized as much.
At that point ease of management or package installation probably matters more to developers and it might simply come down to things like driver support and other stability/performance issues where Linux has gotten a LOT of highly-specialized attention from hardware vendors and the HPC world. Back when 10G hardware was just starting to enter the market, we bought cards which came with a Linux driver but it took awhile before that was ported to FreeBSD and longer still before the latency had been optimized as much.