One reason you might find .280 ammo more easily there is it was originally designed for use in pumps and semiautos, as a lower-pressured cartridge providing .270-equivalent ballistics. Pumps and semiautos have never been as popular in the east as out west, and even though there may be more bolt-action .280's in the east these days, there are probably still a lot of pumps and semiautos around.
I don't remember anybody here in Montana owning a .280 until it became an "in" cartridge among people who had custom rifles built in the 1980's, after Jim Carmichel started promoting it in OUTDOOR LIFE, and other people started promoting 7mm bullets as providing extra killing power, not explainable by foot-pounds or other statistics. This was long before some hunters started shooting big game well beyond 500 yards, so this wasn't due to the higher BC of some 7mm bullets, but something magic in the 7mm diameter. (Which is, as Bob pointed out, the actual diameter of .270 bullets.)
These custom-rifle loonies mostly chose the .280 for the same reason they chose 1909 Argentine actions, French walnut and other special stuff, because they wanted to set themselves apart from the crowd. I can even remember one of these leaning over the counter of a local sporting goods store, talking to a clerk who also believed in 7mm magic. They bonded like a couple of fans of obscure single-malt Scotches.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.” John Steinbeck
