I know with RTRs, they try and keep costs down to have a more attractive price tag. Including plastic hexes vs. machined aluminum hexes... a company just saved a good chunk of change right there.
It's also quite common to make the move to metal hex adapters if using a high powered system with...
Your new wheels... perhaps while in the mitts of tearing down and putting back together your RC, you have forgot something like a crush tube. Maybe the new wheels are not quite like the old ones, and have an extra bit of plastic compressing on the bearings, or steering block as you tighten down...