I can't speak for all clubs and ours has never hosted a Sunday event, but getting facilities for Sunday tournaments is a little tricky. Our school district (and I suspect most others), has a long standing policy against the facilities being used on Sundays. In recent years that restriction has been eased some. We could try to obtain permission to have the facilities, but in order not to rock the boat we would likely host only small novice tournaments at first.
Another problem is there are already too many tournaments each weekend. This doesn't allow clubs to make much money or even cover their costs of hosting. If you added a tournament on Sunday someplace that would only further deminish the Saturday hosts profits.
Then we have the problem of the parents who are not looking out for the best interests of their child and would be going both days to different tournaments.