That requires taking resources from building a fleet able to actually fight Gaurdians, Ships to survey, slips to build the above, automated mines for the civ econ to ship to asteroids, more fucking missiles(which a PDC will also need), mass drivers for comets, civilian ships to move shit, the transports for moving shit before the civilians can do shit(infra mostly, but I tend to make them early on the main troop transports and eventually give this job with a back up job of being emergency freighters).
Only have so much resources and time. And when you are struggling to find new sources of the rare resource called duranium or gallicite in your first game, a PDC is not high on your priority list.
That game was fun. Did you know it is hard to do shit when you have low ability to aquire the two resources all ships need?