Some password hashing algorithms have a maximum input size. For example, bcrypt is limited to 72 characters. The speed of password hashing (which is deliberately designed to be slow to impede brute-force attacks) might also significantly depend on input length, hence overly long input may take too long to hash. Other than that, and general limits on request size, there is no good reason. On the other hand, since the hashes are usually fixed-length, you also don’t gain anything by using overly long passwords.