I would guess two reasons :
It wasn't the practice in those days to give knigthood to a mere sportsman. If you look at that list, till Bradman, there weren't any who were knighted for playing ability alone. I don't know about other sports, but in football too it was in the 1950s that a player was knighted.
WG Grace was a commoner, and the class differences were given a lot of weight in those days. Going by the cricketing defintion WG was a 'gentleman', but he was from the middle class. All the knights before Hobbs were aristocrats.
[Edited on 9-10-2003 by Tintin]