Linux added differentiated support for high-scale and high-performance systems pretty early on. Good support for very high core counts, kernel bypass mechanisms, XFS, and more recently things like io_uring. This opened Linux up to a class of high-end applications that FreeBSD wasn't designed for.

FreeBSD was excellent for ordinary UNIX-y server things. If you were designing a high-end database engine though, where you mostly just need the OS to get out of the way, it was much easier to target Linux.