1. Immutability: Functions are expressions that return a singular value without mutating state. Most loops are not expressions and have some mutation happening.

2. Composition: If the loops don't mutate there are functional alternatives (map, reduce/fold, filter, etc) which are composable.

3. Abstraction: Loops are traversal instruments. Sophisticated functional languages / environments have some mechanism for generalizing traversal of any data structure into a higher order function or some form of reusable generalization.

That said, I use a lot of loops in library code for performance and expose functional interfaces for consumers.