A couple of reasons:
Firstly, in my country every time a game mode is split, ranked, unranked, events, normal etc. it increases the queue time for each mode, as there is less players in any one mode. Queue times have exceeded what players, on average will accept (e.g., players not willing to wait >5 minutes for a game) and so they stop playing which compounds the problem until the mode is no longer active. In that respect Ranked modes no longer pop in my country but normal pops instantly. Map selection causes a similar problem where is splits the player base as well.
Secondly, I don't actually believe there's a different between ranked and unranked. If you want to have a chill game then don’t try hard, if you want to play more competitive than indeed try hard. The issue is that players want to be able to have a 'chill game' and still go well by not vsing hard players. To me that doesn’t make sense. If they combined ranked and unranked, the players not interested in the ranking would still be able to play, they would just be in lower ranks.. And the players interested in playing more seriously would still be able to play, just in higher ranks, and they may have some lower rank players in the lobby.
