Is there a really good reason for why I gotta play against diamonds in norms?

Normal MMR and Ranked MMR are separate, so you shouldn't compare them.

So? I'm Diamond as well, but when I play a normal I sometimes even meet Silver players. Normal matchmaking work completely separate from ranked matchmaking.

That's why low elo players trying new stuff in ranked rather than in normals/flex or you can make smurf to try new things, but since you must buy all champs it sucks. "nOrMaL mMr Is SePaRaTeD fRoM RaNkEd OnE" -unknown hero True question is why tho...

The reason is queue times. If I played on my main account in norms my queue is consistently 15+ mins, so it extends the search enough to include shitters.

Normal games are actually TERRIBLE. People complain about one-sided Ranked games... wait until you try Normals hooooolyyy f***. Legit every game at least one lane is 0/6 by 5 minutes, and the game is "over" by 10 minutes every single time.

I used to try to practice in Normals since I don't have a second account, but recently the game quality has been consistently so poor that I just play Ranked only.