Why should I not reuse a password?
Answer first
Because one leak is enough. The same password gets tried by machine against other services, and one service's breach then costs you several accounts.
A service can suffer a data breach even when the user has acted carefully.
Leaked usernames and passwords can be tested automatically against other services. Reuse then opens more than one account.
Even a long password does not protect other services once that same password has leaked from one.
A password manager can create and store unique passwords for you.