Back when the choices in doubles were pretty much limited to short barrels (usually 26") with open chokes or long barrels (usually 28") with tight chokes, economics played a role as well. That era, for the most part, was prior to the popularity of screw-in chokes. So, in terms of economy of scale, if you're making a whole bunch of short barrels, you save some in the manufacturing process by boring the chokes all the same; ditto the long-barreled guns. Now that most OU's (and even quite a few sxs) come with screw-in chokes, all you have to do is thread all the barrels, regardless of length.
You must be talking about barrel lengths in new guns, Larry. There has never been a shortage of vintage doubles with 30-32" barrels, at least since the early 1900s. All these vintage 30" doubles that you see for sale now with tight/tighter chokes were around 50 years ago too, just cheaper.
SRH
