Mostly just because I've never used FastCGI before, and I have played around with making C/C++ servers for fun before. Seemed like the fastest solution and so far its worked out better than expected.
This is a bit of an oversimplification, but where CGI spawns a new process for each connection, FastCGI* starts the process once, then runs a loop to handle each connection, so the process startup, database connection, etc. costs amortize to essentially nothing -- many of the constant factors for working in a higher level language are eliminated.
FastCGI is worth looking into - a lot of popular webservers support it, and it's less of a complete model change than switching to an event-based system (e.g. node.js) or an MVC framework.
* Or SCGI, which is a newer, simpler design with similar goals.
