I'm not sure but my guess is that it has something to do with muzzleloading laws in some places requiring a .50 and someone carrying over that mentality to slugs. Something to do with the rate of twist in barrels that originally shot close to full bore slugs of various designs. And something to do with the fact that the companies who developed sabot loads not producing elongated bullets but having lots of pistol bullets on hand.

I'm with you though, I'd like to shoot an elongated slug through my 210F but would have to handload them. I just haven't taken that plunge yet but have really considered it.

You're talking about sabot slugs in shotguns I assume. I imagine this is for places where rifles are not allowed, like in built-up or low game density areas. It would give you some reach over plain slugs.

It sounds like an adaptation from muzzleloaders. The .50 became the standard in muzzleloaders several decades ago, and at the same time sabots have become standard in modern in-line muzzleloders. You put the two together and that's probably it. It's what shooters expect. Marketing in other words.

However, in the '70s and '80s, .45 in modern replica muzzleloaders was quite common for hunting.