There's also the population thing (as well as con, unemployment and stuff like that). France, for example, fills up every colony it gets with franks in no time because they have a lot of main culture pops that have nowhere else to go. Russia actually is the best at filling up colonies... they start with HUNDREDS of them (pretty much all their non-european provinces, no matter if they start off as colonies or not in vanilla ahd) and turn them all russian within a few years. You can't expect to fill up even more colonies where there's so many empty provinces for them to go already. Turkey has fewer main culture pops and some relatively empty provinces in the balkans for their pops to immigrate to as well.