That's because most people like a heavy, fast hitter. Sweepers like Sawk are more 'usefull' ingame because OHKO most things, making it easier. Throh would take a hit first and then kill them; which is also good (revenge, vital throw, etc), but takes a toll after a while (let's say a long route).

Also, if IIRC Sawk and Throh are not version exclusives. You can find the other one in the shaking grass. Very rare, though.