Mesa's main thrust has always been guitar amps and cabs. They dabbled in bass gear over the years but, with the exception of the 400 and 400+ it's only been in recent years that they got real serious about bass. I suspect their income from guitar amps far outpaces that from bass gear and, for many dealers, there aren't enough bass players interested in Mesa to justify carrying bass gear. It's rather expensive with the exception of the Subway, which is a great bargain.
The very first Mesa amplifier was a bass amp. They've been a bass amp company from the beginning, just the guitar stuff is where the real innovations happened and of course that was their real growth area. But they're as experienced in making bass gear as anyone else.
Also most class D amplifiers are cheaper than normal ones for some reason (compare prices on Aguilar's Tonehammer with their DB751).