I think you misunderstand the purpose of debates.

Actually they aren't even really "debates" in the technical sense they are question and answer sessions. What they are for primarily, I think, is to get some serious political Q&A into prime time so that all the Americans who can't be arsed to go online or watch speeches can get some exposure.

I mean people what this stuff in bars.

The secondary reason is to get a chance to hit the candidates up with some tricky questions.

Extending the time puts it back into the tl;dr catagory. And on top of it showing them would no longer be profitable for the network.

And yes Libertarians could complain about matching funds. But that's a seperate question.