[Lil' Lavery]

A majority of successful teams use some variant of scrub, skid, or tank (they're all the same thing) steering every year. This is probably because they are the simplist and by far the most numerous drivetrain used in first. Every single FIRST Champion has either used scrub steering or a swerve drive (or a combination of the two), with all but 2003 Wildstang and 2005 HOT using scrub steering. The amount and type of wheels/treads vary from bot to bot. No holonomic, ball, or mecanum drives have ever won the Championship event, but that may also be because of the relatively low number of teams who use these drivetrains.
2006 was dominated by 6WD scrubs and treaded scrubs. Because of the wide open field, yet relatively small scoring area for a majority of teams, and the steep inclined ramp, both pushing power and speeds were premiums (even more than other years). Rapid precision turning was not as necissary as other years, especially considering many top notch bots could accomodate for small variances in direction with turrets. Thus the dominance (and frequency) of high traction drivetrains on many bots.
Many teams which had built swerve drives (which enjoyed a fair amount of success this year, especially by teams 71, 118, and 1261), but several teams that had used sweve drives in the past used a variant of scrub steering this year (such as 67, 217, and 1114).
Several other teams used scrub systems that were better at turning, but had significantly less traction, such as 494/70, 1038, and 1625.
A few teams had notable success with mecanum drives, such as 40 and 357 (who both advanced to regional finals), but I do not beleive any mecanum drives won events. Holonomic drives saw a notable decrease in usage this year, as they could not make it up the ramp without a very advanced suspension system, so there wasn't much success from them in 2006.
Widespread success of drivetrains typically results from a larger quantity of teams using them. Because a majority of teams used scrub steering in 2006, they had the most success.