When pubs first reopened I was mostly visiting Sheffield pubs, many of which are associated with or owned by a brewery: Sheffield Tap/Tapped Brewery; Fat Cat/Kelham Island Brewery; Raven Inn/Loxley Brewery; Kelham Island Tavern/Blue Bee Brewery; Blake Hotel & Wellington/Neepsend Brewery. These pubs were mostly selling just their own beers, with maybe one or at the most two guests.
Partly this is because they could keep control of the supply chain, which was important when business depended on the weather (between 12 April and 17 May) and how their customer base would respond to the relaxation/weakening (depending on your point of view) of the Covid-19 restrictions. Also for the owners it would mean that their breweries were making money as well as their pubs! From conversations I had with some of the managers/licensees, they were wary of putting on two many beers and having to throw them away when they either couldn't sell them through lack of custom or restrictions were reimposed (which didn't happen, of course, but it could have). An exception to this was the Stancill Brewery's Closed Shop, where they had a full range of their beers on in April; I tried three - one had a brewing fault and the other two were dull. (I've not been back.)
More recently, the Sheffield Tap has had a greater selection of beers, and I think last time I was there they had a beer on every pump (except for the cider!) and only three out of nine were their own beers; this may be the case at the Kelham Island Tavern, but I've not been there recently (also they were closed because of Covid among the staff). The two Neepsend pubs have had fewer guest beers on than they did before March 2020, and there's usually one hand pump unused in each pub. The Fat Cat was celebrating its 40th Anniversary this weekend, and had a mini beer festival. There were (I think) 10 beers on the counter in the pub, and there was another bar in the beer garden (in the small building where the original brew kit was situated) which I didn't check out. Once those have run off, I expect they'll be back to five or six, as before, and mostly their own.
In Leeds, where I go for the rugby league, I've been going in the Cardigan Arms and the Scarbrough Hotel. The latter is a Nicholsons pub, and they've had three or four beers on mostly, whereas before I think they'd have had five or six. I noticed last Thursday that the Tetleys was doubled up, which I can't remember seeing before. The Cardigan is owned by Kirkstall Brewery and is selling almost exclusively their own beer - but I can't remember seeing many guests there before (a couple of Hawkshead beers one time - but maybe that was a swap). Last week they had one guest keg beer: that was it, all the rest their own.
Another Sheffield pub: the Queens Ground. Lovely pub, used to have four hand pumps on, built up from nothing only seven or eight years ago when I reviewed it. In May when they reopened they had two beers on - because, as the licensee told me, he didn't know how the trade would be and until football restarted he didn't think he'd be busy enough. Sure enough, first Owls home game I was back there and there were three beers on the bar with one just run off.
So to answer your question, Tris: I think some of the breweries that don't either own pubs or have a distribution agreement with a pubco or wholesaler may have been struggling for orders. And pubs that have been able to tap into a steady supply from major breweries may have decided that they don't need to bother about some of the smaller breweries any more.
Another thing that may be coming into play is that it's been noticeable that beer quality has been more reliable. I think I've only had one or two beers since 12 April that were stale or slightly off, and maybe four or five that had brewing faults (plus the three mentioned at the Closed Shop). That's less than a dozen poor beers out of 185 pub visits and just over 300 pints. Other than that it's been perfection, perfection, perfection. So maybe the penny has dropped: fewer beers means better quality means happy customers drinking more beer means less wastage means more profit.
And maybe those drinkers for whom variety and innovation are the thing should remember Camra's Revitalisation slogan: "Hey, some of this keg beer is actually pretty good, isn't it!"*
*(c) Sheffield Hatter. (Just kidding, guys. Please don't sue me.)
Sorry, that was a lot longer than I expected. As the actress said to the bishop. I'll get me hat.