I think it was probably because BSA were more "savvy" to the bell target market. Their products had been market leader in that field for years before WW2, so they were probably better placed to notice the down-turn in popularity of bell target as a sport. Webley came late to the game, trying to corner the market with the Supertarget, but bell target was already on its way out by that time, as was 6yd competition. By the middle of the 1970s, bell target and 6yd shooting had given way almost entirely to 10m matches, best shot with recoilless rifles and pistols.
I think BSA got out while the getting was good, but Webley just didn't read the writing on the wall.