Yes, there is a later flight, but it doesn't arrive until like 9:30 PM. If they are guaranteed a nonstop on Saturday, it might not be worth it to arrive late on Friday evening. Plus, they'd have to pay for the over night themselves.

Regardless, the principle is don't necessarily just accept whatever they give you, if you can find better alternatives that suit you. One time our Delta flight from Tampa to Atlanta was cancelled that morning, and we were able to get them to switch us to US Air and actually got in that same day, earlier than we would have originally.