That's a great question and one I've asked myself. The answer might be that WABE has the heritage and hasn't given people a reason to switch to WRAS.
When GPB first took over WRAS, WABE had a signal advantage. Both stations were 100KW, but WRAS was on a short tower in Panthersville, and WABE was at 1,096 feet from New Street in the heart of the market. But WRAS was able to relocate to the Chester Avenue site, much more centralized than Panthersville, and increase HAAT to 1,043 feet. By doing so, WRAS had to half its wattage from 100KW to 50KW. But except in the fringes of the market, both stations have about equal signals.
WABE has had great morning hosts with Steve Goss and than Denis O'Hayer, both very well known in the market. Now that Denis has retired, whom they bring in has yet to be determined. I doubt, however, that the local hosts have that much to do with the ratings (except for Lois Reitzes and Rose Scott, who do local shows). WABE also has an excellent and large news staff.
WRAS does programming for just Atlanta that is not carried on GPB throughout the state. I don't know the size or quality of WRAS's news department, but they certainly have one. Former WSB-TV reporter Bill Nigut is very visible over there.
At this point I doubt WABE listeners have any rancor towards GPB regarding WRAS. They might just have no reason to leave WABE.