1. Price - 800res compared to alternative 300 res for gun platform. And platforms have much better accuracy, the only thing refineries really do work against are unprotected resoursing operations. Put some vette/platform screen and those expensive refineries go in flames.
2. Build time - again much longer then platform's time.
3. Effectiveness/accuracy - only good vs unupgraded/pass starting fighter class and collectors when they sit on the asteroid. Gets eaten alive by anything else, especially when outnumbered (look at the resourse cost). Only a suicidal person would send interceptors to counter a heavily armored ship, even if they don't get hit, they simply won't do any significant damage.