Around the web, "epic" consistently describes the Nadal/Djokovic 6-hour marathon match in the Australian Open final. The effort, stamina, and skill of these competitors deserves admiration. The final followed on the heels of a nearly 5-hour semifinal between Andy Murray and Djokovic. A 353 minute match? That's 203 minutes beyond Bill Simmons' suggested 150-minute limit for almost anything (See "Less is More"). For me, even the Murray-Djokovic match went from epic to ridiculously long, and I didn't start watching until the third set.
Why does men's tennis use a best-of-5-set format in grand slam events? The quick, Econ 101 answer is that it must be profit or revenue maximizing. I'm willing to be proven wrong, but my default position is that 300 minute matches don't maximize viewership in the short run or long run. Sure, there are hardcore fans that may stick in there, but I am the marginal tennis fan. I watch quite a bit of women's tennis and very little men's tennis for the very reason that I enjoy a 1-2 hour match such as Clijsters-Azarenka, but at 2 hours, a competitive men's match has barely started. I once referred to the NBA, where hardly any game matters. In men's tennis, hardly any set matters in terms of sets played where both players face elimination.
A longer version of the Econ 101 answer might go like this: fans like to see the best players matched up; 3-set matches lead to more upsets and fewer semis and finals between highly-rated players, and therefore, less viewers. Ok, Matchups between highly-rated players in semis and finals raise viewership, but that a partial impact. Fan enjoyment of upsets and uncertainty offsets it to some extent, and match length certainly pulls viewership the opposite direction. Why not play 9-set matches? For that matter, why not make tennis like cricket and extend matches over 5 days? If 5 sets over 6 hours is epic, wouldn't 25 sets over 30 hours reach "mega epic."
My guess is that the answer goes beyond Econ 101. Political economy enters the picture. Why does FIFA play the World Cup in South Africa and not only endure but sometimes seem to promote faking. (FIFA: Money, Control, or Both). Yes, the FIFA aristocrats like money (that's obvious) but they have other interests and internal disputes between, for example, the English Football Association and continental associations. Four different associations organize the tennis grand slam events. All of them care about money, but all are also subject to a variety of internal politics and by somewhat aristocratic organizations, with the exception of the Australian Open.