KDF must produce results that have certain randomness properties, and be very difficult to reverse. Password hashes only need to satisfy the property "difficult to reverse", without randomness requirements. This is why all KDFs work as password hashes but not the other way around.

Key-stretching key derivation functions must produce results that have certain randomness properties, and be very difficult to reverse. Password hashes only need to satisfy the property "difficult to reverse", without randomness requirements. This is why all key-stretching key derivation functions work as password hashes but not the other way around.

Note that there are also key derivation functions that are non-stretching. Stretching functions are inherently slow, and this is necessary for password hashing. Fast key derivation functions such as HKDF are not suitable when the input has a low entropy, for example a password, regardless of whether the goal is to derive key material or a password hash.