Coming from other competitive stuff like fighting games n MOBAs, I'm used to consolidation of any kind usually meaning watering down complexity in favor for mass appeal but it makes sense here.
Especially from the competitive side of things, giving sub factions even more options isn't a benefit either. You could have 20 detachments for a faction but it's meaningless if 15 of them aren't ever worth running and 1 is still significantly better than the other 4.
This isn't even too much of an exaggeration either since snowflake loyalist chapters run into this. Blood Angels have a 50% win rate competitively. Perfect, right? They have access to the main 8 normal space marine detachments plus an additional 4 specific to the snowflake subfaction, so 12 total.
So of the 12 detachments available, only 6 get play(this is from the 2nd half of June and first half of July), and half of those are basically statistically insignificant, with an obvious favoring within the top 3. And this is after multiple "nerfs" to prevent snowflake marine chapters from getting the main space marine army rule(oath of moment, basically people were playing "blood angels" with no BA units or characters) in full, while using their own detachments in conjunction with it(leading to some lame broken combinations). It's the same shit for space wolves, dark angels, black templars, and deathwatch.
Now why this is even an issue in the first place is because GW is fucking retarded. CSM monogod armies don't get to just pick generic CSM detachments and units. It makes sense for Ynnari and Harlequins basically just being a half dozen or fewer models(and fits the lore as well), but for the snowflake marine chapters they just get everything. It even applies the other way around so if you're a space marine player with a "codex compliant" army, as long as you aren't using chapter specific characters(Calgar, vulkan he'stan, etc.) you get 24 detachments to pick from for rules.(soon to be 27 once the black templar codex comes out). It's a nightmare to balance, and at the end of the day is just a meaningless amount of "choice".