Every hosting provider has to decide what to provide for their customers on their servers, and the 3 main deciding factors are:
- Do a lot of customers want it?
- Will it be safe for us to provide?
- Will it add any admin and/or support hassles?
AFAIK, not a lot of shared hosting setups provide mod_fcgi. If mod_perlite answers those three questions better than mod_fcgi, then it will be worth it for them to provide it.