Some good replies and some jokes replies up in here…
It was counted at 140 to begin with because most dance music (as of the 1970s) was counted off the kick and had come to center around 133 or so as being the ideal club bpm to keep dancers at max frenzy without tiring them out. That BPM consciousness carried over into nearly all club based dance music.
The genres that dubstep came out of were also aware of this BPM legacy and 2-step garage was already working at 140. When dubstep started rearing its ugly head, in an attempt to add depth and weight (heaviness) a la dub reggae, the downbeat got halftimed - and it works.
In fact I’d go so far as to say it could stand to get even heavier… 100 bpm sludge ftw lol.