If I recall correctly, wasn't that because WinModem was a software modem whereas USR modems were hardware modems? The WinModems were a glorified single chip on a board (analog <-> digital converter), with all the actual work being in the Windows Drivers.

Which on modern computers wouldn't be a big deal (see modern onboard sound) but back then a slower computer actually could mean slower data rates.

There was a way to get the winmodems working on Linux. Lots of fun with configuring obscure drivers, I/O base address, DMA channels, etc. https://tldp.org/HOWTO/pdf/Winmodems-and-Linux-HOWTO.pdf