Have you ever thought that maybe they desperately needed a new Metroid character?
Ridley, Space Pirate(SNES), Space Pirate(Prime), Space Marine(can't remember their exact name), and the four bounty hunters from Metroid: Hunters(DS). They couldn't have been
that desperate for a Metroid character. Perhaps they were more desperate for something that can sway more of the "Smash Bros. is for Kids" Halo 3/Sony crowd over. Play as Zerp Suit once and I can garauntee they won't say too many negative things about Smash Bros. for a while.
This should be highly unnecessary though, the gameplay itself speaks volumes.
You can argue for Ridley, but think about it. Ridley would likely have taken a long time to put into the trailer, he seems like the character that would take some real time to develop. I think that Zero Suit Samus was just easier and more convenient to showcase as a new Metroid character in E3 2006...
Mmmm, no. Zero Suit Samus seems quite intricately detailed, and thoroubly animated(although I probably should look closer to certain body parts to confirm this), there's no doubt in my mind that they couldn't have shown off a Ridley character instead.
Besides the already said reasons, I feel Zero Suit Samus is merely a small piece of Brawl, and was more shocking than an already fan-expected Ridley. However, in 7(?) hours from now I'll know.