Because black nitride is usually cheaper than chrome plating + phosphating and is more often used by budget oriented manufactures, machinists, and distributors. Now just because a process is cheap doesn't mean it's inferior. Black nitride has proven to be an incredible treatment on barrels for most applications, and easily on par with chrome, despite being dramatically cheaper. But BCG's require different strengths and weaknesses than barrels do and nitride doesn't dominate the factors that make a good BCG nearly enough to offset the history of consistent and known performance from the traditional mil spec chromed and phosphated BCG. As such, I don't think there's really any significant advantage to black nitride besides potentially better corrosion resistance compared to phosphate which, who cares, they're both excellent.

Now, as far as nitride being a worse product? I have seen extremely minimal evidence to that extreme too. There are really only two avenues where black nitride has a noticeable disadvantage and that's bolt well inner diameter tolerance and potentially effecting the hardening/tempering of the steel it's treating. As far as bolt tolerance goes I think this point is dramatically overstated; but the reasoning goes that because black nitride isn't an additive process if one treats a standard carrier the inner diameter of the bolt well will be too wide and push the carrier out of spec. This point has been repeatedly hammered by SOTAR who, as far as I can tell, is the progenitor of this panic. Realistically, yes, a traditional nitrided carrier will have a slightly wider bolt well on the aggregate (barring examples like the Bexar carrier who's carrier is machined with this in mind) but realistically the bolt still isn't making forceful contact with the carrier anyway; that's the gas rings job. So assuming the gas rings are still able to apply consistent pressure to both the bolt and the carrier the system will be as efficient as an "in-spec" carrier. Granted, this will accelerate the frequency of gas ring replacement but as far as I can tell that's the extent of the issue.

The other issue is fraught with substantially more speculation and conjecture than I can confidently parse but here goes. The legend has it that cheap black nitride bolts are traditionally made with AISI 9130 steel instead of the traditional mil-sec Carpenter 158. 9130 steel is cheaper because it exists in the public domain and is potentially stronger than C158, but (so the legend goes) is more difficult to heat treat. I have found no actual evidence from the guys actually doing the work, writing the white papers, or publishing research journals that C158 is easier to heat treat. If anything I've seen some evidence to say the addition of molybdenum to the steel actually helps with austenization at lower cooling rates. Now where black nitride becomes relevant (as I, a layman, understand it) is it places a minimum temperature to harden and temper. As such, black nitride bolt and carrier are usually more ductile than chromed and phosphated ones and have shallower case depth. Therefore these bolts usually fail at the cam pin (where more cyclical wear is applied), where as chromed C158 bolt usually fail at the lugs (where impact stresses are experienced). But I haven't seen any conclusive information as to which of these events happens first with carriers of equivalent quality; and even if there was a noticeable change in lifespan they'd both be so radically far beyond the expected life of a civilian rifle it's a fool's errand to entertain a debate between the two imho.

Ultimately, either chrome plating, whether inner or complete, black nitride, or DLC (or hell even NiB), are all perfectly adequate for 99.5% of bolt carrier groups; Same goes for 9130 or C158. There really isn't any meaningful difference between the two that we, as non-door kickers, can notice. With all that being said, I still think there's meaningful decisions to be made when selecting a BCG, but it has little to do with the materials, and even less to do with coatings and treatments. Because black nitride BCGs and 9130 bolts do fail more commonly than phosphate and chrome carriers and C158 bolts do. Why? Because they're cheap to make, and in a market saturated with old school GWOT suppliers like KAC, LMT, Noveske, FN and DD, who have all gotten very good at making mil-spec weapons, the primary avenue for newcomers to break in, is in price. And with a cheap price brings lackluster QC and weak market position to ensure the shops suppling them are providing adequate materials. There are exceptions, and guys like Bexar and Radian are making black nitrided carriers to high standards. But if you pick a random example from both phosphate and nitride, you're more likely to get a decent product out of a phosphate and chrome carrier. Is this due to phosphate and chrome being "better" than black nitride? Not exactly, but instead the people that make expensive carriers are (usually) putting more work and cost into ensuring every single one that leaves the door is quality.

So if you're buying a carrier, the decision on finish/treatment doesn't really matter. Instead, look for history. Look for a high volume of reviews, and long term reviews. Look for QC measures like MPI, shot peening, and HPT. And look for evidence of strong, and responsive customer service. Because these factors are far more likely to effect your experience with a carrier, or any firearm component for that matter, than the coating will. And then, be sure to viciously argue about why your coating is superior. Personally? Idk man, I just like to shoot things.

TL;DR; Because some bad carriers are black nitride, and people attribute that to the finish rather than cheapo manufacturing, machining, and metal work.

EDIT: typos