Different mindset. Nowadys there's strict usage criteria defining what's OU and UU, and BL is purely a ban tier. The subjective "as far away from OU as possible" criterion for the tiers is not really used in these decisions, so BL is kept small. That's not the way it was seen in the RBY-thru-ADV years.
In the early days, BL was a place for things that were close to being OU, but not actually "OU". Basically, things thought to have OU power but not OU usage. UU was then set as "everything else" that was as far from OU as possible. A couple overpowered UUs were then sent to BL afterward.
Plus it doesn't help that Smogon's GSC UU and BL tier lists were mostly theorycraft to begin with, since UU wasn't a very highly-played tier.
I saw your GSC UU thread in Ruins of Alph, and since I wasn't around back then, I have a question.
Why are the old gen UU lists set so far below the OU ones? It seems strange to me that so many Pokemon just get put in BL and never get used in any tier very often. I take it this was just a different mindset back then but it got me quite curious.