Because the web operates on standardized protocols, and the use and function of the "www" prefix is a defined and well known part of the protocol. It's got its

Because the web operates on standardized protocols, and the use and function of the "www" prefix is a defined and well known part of the protocol. It's got its own dns record, and can be functionally different than a bare site. Google did this in Chrome specifically to enable fuckery with Amp and hide shady behavior. Their own page ranking penalizes sites that don't use the www prefix. They know both situations are valid, they just found it easy to throw smaller site owners under the bus. However bad Google is, though, the real culprit is the OP's host. They're not giving users the correct tools to run websites.