It had a lot to do with the cost. G bugles were getting more rare with the ever advancing Bb/F instruments used in modern bands...In fact now I think Dynasty and Kanstul are some of the only places you can still even purchase G Bugles.
Corps realized hey...if we switch to Bb we now have something that is worth a resale value. (notice how all the corps resale their instruments after a season or two so they can repurchase.
Once that happened, a lot of the smaller corps and DCA corps started buying old DCI top class G bugles like hotcakes, lol.
Also, lets not forget that Bb/F instruments are SO much better in tune. (uh oh, i just gave away my age as to being younger!)