When I started writing it back in 1999, it was what I knew. I also had a C library for dealing with CGI scripts I wrote from the mid-90s. Also, for several years my website was run off a 486 I had colocated at a company my friend was running, so there was a performance issue as well.
Because it's still the best language for writing library routines along with a header file, which then can be used from most other languages with least runtime overhead.
[1] http://boston.conman.org/
[2] https://github.com/spc476/mod_blog