I think it's a combination of factors, mostly connected to the fact that 4 and Melee are, to an outsider, take up a similar space. I know a bunch of Melee fans just ripped out their head in anger at that statement, but to a layman, the games seem similar. So I think they want to avoid overlap between the two. Not just with viewership, but also I think they were thinking about player entrants. It was safe to assume, even though a lot of players only play each exclusively, there probably is a lot of overlap between the two in terms of entrants, so to make it easier on them (less running around the same day to play both games), they'd spread it out. Granted, it turned out funnily enough that there's actually more overlap between SFV and Smash 4 than there is between Melee and Smash 4, but eh, same point applies actually since SF finals are also Sunday.

So yeah, I think everyone behind EVO know that obviously Smash 4 is a bigger game than Pokken. Hell, it's second only to SFV. But they just didn't want it to share the same day as Melee. So actually, the real question you should be asking is, if that's the case, why isn't Melee on Sat. and Smash 4 on Sunday. The traditional response has always been in terms of stream numbers, Melee has drawn more, but I think that's actually slowly changing with Smash 4 gaining momentum. Frankly to be honest, I think Melee gets the spot based a lot on its legacy. It's the 15 year old game that wouldn't die. I think a lot of OG Melee players would start a riot if they're spot was switched with Smash 4 because they feel they've "earned" this through all their years of keeping the scene alive. Now whether that's a good enough reason to keep the spot, we'll I have my own opinion about that, but in the end, it's up to the tournament organizers at the moment, until the masses tell them they want things different.