Probably another "player retention" thing. Respawn probably calculated that the "joy" and smurfing brings to player base as a whole greatly outweighs the "frustration" that a non-smurf feel when getting killed by a smurf. In other words; allowing smurfing makes more people keep playing the game. Because if Respawn wanted to get rid of smurfing, they should have done something about it. Now, the system has not even a single feature that makes it harder for people to smurf.
Raising the min level requirement from 10 to maybe like 50 is the first thing that comes to mind. It wouldn't eliminate smurfing but at least makes it harder. Because honestly, I see NO reason what so ever for an actual and genuine level 10 player to play ranked so early in his run with this game. Only smurfs play ranked at lvl 10.
Also, a "fast advance feature" would work too, and is used in other competitive games like League of legends. This means that if the game detected that you are steam rolling everything and everyone in your matches at your current rank, you could gain several rank-ups to a level that better matches your skill level. I dont think many Diamond players would bother to smurf in Bronze if he got fast advanced to platinum in just a handful of games.