I don't think Mach Rider got the most votes, but Mach Rider was considered for Melee, so perhaps Sakurai felt like adding a retro he had previously considered back in the franchises earlier days.
I doubt the Mach Rider placed within the top ten or possibly even the top twenty overall. Any of the Retro characters from the NES era (Mach Rider, Takamaru, Ayumi Tachibana, Balloon Fighter, Excite Biker, Etc.) probably didn't even come close to the top ten, however, in order to stay with tradition Sakurai probably wanted to place a Retro character in the roster anyway.
Looking back at the mock polls during the Smash Ballot days, the character among the Retro category with the most votes was either the Excite Biker or the Mach Rider. The two characters are similar, however, the Mach Rider is more combat oriented so Sakurai decided to go with the Mach Rider thinking the Excite Biker people would be pleased with the Mach Rider as well.
For this reason, I'm thinking the Excite Biker will never be playable in Smash. The closest he will ever be is a skin swap for the Mach Rider.