Sakurai probably picked Toon Link again for two reasons:
1) WW HD
2) It might also have to do with Zelda Wii U
why do I say number 2 you ask? Well, take a look at this picture:
Notice that there is un-familiar island that has never been seen before? What could that island be? Plus if you look directly at Link's belt, it has two lines. You should also notice that Toon Link seems a bit more mature this time around. Also, it is rumored that if you beat WW HD Hero Mode 100%, you get a sneak of Zelda Wii U. Could Zelda Wii U really be another direct (or another in-direct like ST) sequel to WW, much like PH? Is it possible that we're seeing of Toon Link in SSB4 is a glimpse of what to expect of Zelda Wii U?
Edit: Well, it turns out, after awhile of looking at the picture, I realized that the island is actually from Wii Sports Resort. When I realized this, I felt kind of stupid for not noticing that in the first place and I pretty was very upset that it nothing to do with Zelda Wii U. Also, what is the island from Wii Sports Resort doing in SSB4 anyways? Is there going a playable Mii Character? Or better yet, are players going be able play as their Mii Characters?
Edit#2: While, it is still possible that Zelda Wii U could be another adult timeline game and appearance of Toon Link's return could relate to that, I only the crossed out the stuff I said about the island.