Historically speaking, module availability has always been intended to follow the local tech level and economic prosperity. Since Colonia is more of a "frontier town" sort of place, they tend not to have the highest grade gear.
For most modules, "B" is still pretty decent, but a lot heavier. "C" grade modules usually weigh the same as "A", but the performance isn't as good. "C" still performs better than "D", which are the lightest (of course).
It can be a hard pill to swallow when you're used to the best. I know.