It splits the population and means neither playlist gets enough players instead of just biting the bullet and combining them which would be more beneficial in the long term even though some players might be lost.
The HCS playlist is still kind of new so they most likely just haven’t decided to merge them yet.
Team Arena would never be merged with HCS. If anything will happen, Team Arena will disappear (less likely)or get a re-branding (more than likely).
The HCS playlist only includes map/gametype combinations found within the HCS format. Team Arena has more maps, gametypes, and combinations. Team Skirmish includes even more, but is also the unranked variant.
The need to put koh and oddball in both playlist strong holds suck if a player leaves gets lagged put dropped from the server and oddball is fun and in social playlist half or more of the players just kill yo pad there k/d and won’t help with the objective