Marketing: save the best for last.
It is simple stuff, but there is a strong reason for this.
String people along (the core fans) to wait until the end for the characters they wanted most, while introducing new fans (those of Little Mac, Megaman, the people revealed already) into Smash Bros.
It also makes the candidates stronger and stronger, rather than weaker and weaker.
Would it have been better to get the fan favorites- Mewtwo, Ridley, King K Rool, first-- and then end with Wii Fit Trainer, Mii, Rosalina, and Greninja?
No. That is resentment and poor planning.
FInish big. Start smaller.