I don't know that there's any single explanation. It's likely a lot of factors. It could even be as simple as production volume of Federal spp's isn't as much as other brands.
I know that revolver shooters like the, easiest to ignite primers in their tuned double-action revolvers, but in this day and age of most people shooting autos rather than tuned double-action revolvers those revolver folks should not logically be the blame for any shortage.
True, semi-auto's are a lot more popular these days. But . . . when you look at the proportion with those who load their own ammo, the balance takes a considerable shift toward revolvers. You show me a guy out at the range shooting a revolver, and I'll show you someone who loads their own - 90+% of the time. Do the same with a guy shooting a semi-auto, and there might be a 70% chance he's loading - if that. This is not to discount or refute your point, however. I still agree with you: folks loading for their race revolvers can not account for the apparent shortage of Fed spp's.
Back in 1988 (1984 for me), I walked in to my LGS and asked for small pistol primers and they handed me CCI-500 and thus... for the next couple of decades, CCI was my brand and no other need apply.
It wasn't because they were cheap, it wasn't because they were "hard", wasn't cause they were "hot"... it was nothing more than because it was what they handed a 16yr old (21yr old for me) new handloader.
I could have written that ^^. Except for the two corrections in italics. Until the big shortage of '13, I never used any other brand. During the shortage, I bought Federal LP's (No. 150) and Match SP's (No. GM100M). They all worked great. But once the shortage was over, I was back to CCI's. Except I use WSP's for my "semi race" Smith 38 revolver I use in competition - but that's for another post.
And when it comes to primer sizes... it should be painfully obvious that small pistol totally rules the roost. .380, 9mm, .38/.357, 40cal and now more than ever... .45 Auto.
This likely accounts for some of the shortage - at least, in part. I realize that wouldn't account for the difference in brands - unless Federals are less expensive (which I don't know). I know when demand is high, the less expensive components are the first to disappear. Point is, SP's are in more demand in general, and so it would be easier certain brands to go bare on the shelves.
For what it's worth, here in my area, there is a shortage of primers in general. But I'm in California - I'll just leave it at that. Good thing I'm sitting on about 20K of them