Because those groups have ways to counter bash protection very effectively.
Empire shaman was a common power build years ago, as centurions did not have nearly as many restrictions on its use and shamans are consistently strong whether fighting alone or in groups, prepped or unprepped. Thus, if you were a bash spec and rolled with an empire shaman, summon, dispel, and damnation would reliably rape almost any character in the game. And you were much more likely to get the last hit than your shaman partner was. I'd attribute a lot of the empire bash spec kills to running with good shaman players who could handle 90% of the common counters to bash.