It’s just a matter of practical convenience to preserve the (admittedly arbitrary) numbers as they are, but it also makes sense for objective scholars to remove the inherent chauvanism from the system for their own purposes. There is also the issue (as has been pointed out already) that the BC/AD system is simply inaccurate as to its central point. If Jesus was born during the reign of Herod the Great then he could not have been born any later that 4 BCE (that’s when Herod died). So why should historians, especially non-Christian historians, be forced to use appelations which not only don’t reflect their own religtious beliefs but which don’t even accurately represent history? The numbers themselves are so entrenched and so universal in western culture that it would make no sense to contrive a new system completely but most scholars these days simply feel more comfortable using the phrase “common era” which accurately reflects the way the west counts years but does not necessarily genuflect to the significance of the central date itself.
Please note that there is no movement by scholars or historians to force anyone else to stop using BC/AD, many of them just don’t want to use it themselves. There are no petitions, no pending legislation, no attempts whatever to codify the phrase “common era” as official or binding on anyone else. You can say whatever you want. Both “Anno Domini” and “Common Era” are merely conventions of speech in any case. You say what you want, I’ll say what I want. The numbers are the same. Like Buck said, there is simply no reason to get offended by the choice that some historian makes.