There's actually a good reason behind hiding player numbers in playlists: People don't want to search in a playlist with a small number of players.
To simplify things, if you see a playlist with 100 players and a playlist with 5 players, most people are going to choose the playlist with 100 players for two reasons. First, it has more players so it must be better. Second, it has more players so it will be easier to find a match. This second reason is important to keep in mind.
When a playlist has a low population (and players know it), they avoid that playlist, which only serves to make the playlist population decrease even more. Why search in a playlist you know will have long wait times when you can search and get a match immediately in another? By removing playlist population numbers, this issue is solved because players search in whichever playlist they want, and so the population of the game is more evenly distributed.