Many reasons, but the most obvious is that I'm not here during the day and he is dependent on the connection for schoolwork. He doesn't know UNIX or Linux, has no interest in learning about it, and doesn't know much about networking hardware or configuration. Our connection depends on a cell phone modem that of course has documentation, instructions, and support only for Windows. If he has a problem and I'm not available, he's much more likely to get it resolved if the connection comes directly to his machine. Verizon support will not help with Linux or with local networking. (Probably won't help with much of anything, I suspect, but at least he feels better about it.) He can reset or restart the connection himself as long as only Windows is involved. In fact, yesterday he migrated the modem to his laptop and got it working without my help, because it is made to work for an idiot on Windows. A Linux box would require some setup actions.

If we had a real DSL or wireless broadband connection then I'd insist on a Linux box as router and firewall, but those services are not available here.