There's a big difference between the 16 Bears games a year and the 162 Cubs games a year. CBS obviously wanted to get in on the Cubs so they bit the bullet and put the Cubs on WBBM, but afternoon drivers who aren't Cubs fans will eventually go elsewhere. They have the FM station but its easier to just run over to the WGN preset on a daily basis than go through the arduous of finding 105.9 on the FM dial and setting a whole new preset that you have to change bandwiths to even use.
It's bad for WBBM long-term to continue broadcasting Cubs day games, it muddies their role. The Cubs are unlikely to have a problem being put on WSCR with an afternoon driver show that deifies them.
WMVP might have the wattage to make it past Aurora but the signal is garbage. The quality blows and runs in and out if you're within 100 yards of a bridge or telephone line. That's not the case with WGN or WSCR, I dont remember WLS being a problem but I wouldn't be caught dead listening to WLS nowadays.