I don't really know myself, probably the many footguns and complexities. Like taking a reference to a value in a vector and then resizing the vector, boom undefined behaviour. There a re a lot of things like this. Maybe also the verbosity and multie ways to do things. Look at the linked code, there is a lot of overhead to the function definitions, sure there is a point to all of it and I put it there for a reason, but to people less familiar to the language it is probably quite offputting.

Personally I like the language, second only to rust for me