I get the attraction to Ponderosa bonsai. They're spectacular. Eastern pines in the wild don't get the exposure at altitude that Ponderosa do that produce spectacular trunks, deadwood, and bark. Eastern pines tend to be wallflowers comparably...
Because they're being collected by professionals in numbers, excellent Ponderosa raw-stock is also more affordable than Eastern and Southern U.S. pine raw stock. You can get a very nice three or four hundred year old jin-shari marked ponderosa with thick plate bark for $1,500 or so. Can't get any Eastern collected pine anywhere near that quality for the same money.
I can count a couple of hundred great Ponderosa bonsai and maybe two excellent Loblolly pine bonsai. Pitch pine is the best candidate in the east for bonsai, but again, it's not common (not as common as it used to be) in bonsai use. Decent collection areas for Pitch pine are limited geographically and legally as well, at least compared to the millions of square miles of collectible territory out west on public BLM lands.