Well, it might partly be what you said and that they simply don't have that many comedy shows to fill out 36 hours a week. And I'd imagine aquiring anime would be cheaper than making their own shows as well. But, it's not like all the animes get bad ratings. Full Metal Alchemist occasionally beats Futurama. Besides, consider the fact that Saturday is a day where a lot of people go out. However, die hard anime fans would be willing to stay in on Saturday. Not to say that if Saturday was all comedy it would get worse ratings, but well, look at Venture Brothers. It's ratings on Saturday stunk, yet on Sundays it prospered. I think anime, which is more of a cult thing, is better suited to Saturdays where die hard anime fans will stay home to watch it.