Why are players given separate MMR's for Normal and Ranked Queues?

I've found that frequently in Normals, high-skill players and low-skill players are getting matched against one another despite their difference in skill level.

I understand that this can sometimes happen because their Normals MMR must be close, but players ought to be getting matched with and against players of similar skill level to them regardless of which queue they are playing in. Players who play mostly one queue are going to be higher skill than the players they get matched with when they cross over to the other queue.

This is a huge problem because for the lower skill player, there's nothing fun about getting stomped into the ground by someone who is vastly superior to them. And for the high level player, if they're playing normals they don't need to be matched against weaker opponents to get a "break" from ranked, they already escaped the pressure of try-hard and LP gain/loss by playing the casual game mode where none of that matters. And if a player really wants a stomp-fest then they can already play "Coop Vs. AI" to fulfill that purpose.

Is this something that could potentially change in preseason? It would be a lot more fair to all players, and lower skill players especially wouldn't have to worry as much about having zero fun because of getting curb-stomped by opponents they should never have been matched with in the first place.