This is the disadvantage of running wider tires on a 15x8 rim. They hold the bead well, but the wear pattern is just as you say. The only thing you can do is air them down more -- but I don't recommend it.
The only reason to run 12.5" wide tires on an 8" rim is for bead retention. I do it for that reason. If your truck sees street only, narrower tires or 9" to 10" rims are needed with 10" being optimum.
Nothing much you can do except rotate them often. When they wear down in the middle to match the edges they will wear evenly after that and more slowly.