I think it's partly a matter of style. BB King never played Chicago blues as defined by the Chess records stars. In the early fifties it was not uncommon to have a blues performer backed by a big show band. BB King is probably the only one still doing it.
As a crossover artist, BB King is sometimes criticized by blues purists the way Louis Armstrong is critcized by jazz purists. Listen to his early stuff like "3 O'clock Blues" to hear how he earned his reputation.