Availability I think. Despite the fact that I've written in a previous forum regarding a lack of range in UK steelbooks, I do think they are increasingly common and as such are gaining a larger market audience.
I think it's simply very addictive, as with all collecting, and when you buy something, or at least when I buy something, I like to buy the best that I can - in this case BR SB, preferably limited print run.
There could be a hint of OCD to it as well. For instance, in my case, once I'd bought a few beautiful SBs, they looked out of place with the rest of my blu rays, so I decided I'd collect them separately. For every blu ray release with a UK steelbook, I always buy the SB if I intend to get the disc.