Going along with what Sam said... (and with a disclaimer or 3)
This is pretty much entirely based on hearsay and looking at/driving our buggies, but:
My understanding is that after Banyan was built and proved to be wicked fast, the mechanics said essentially 'thats great. now we have a chance to experiment, maybe make the whole design better' and Bristol happened. Im sure there are a lot of nuances I'm missing because I wasn't actually here at the time, but they tried a lot of very different things in Bristol. Some worked, some didn't, some kind of worked in practice (compared to great in theory) but were so hard to maintain or deal with regularly that they didn't improve the overall buggy much.
The next year, seeking largely to avoid the pitfalls eventually encountered with Bristol, they designed Bedlam. (notice how Bristol's front end is rather chipmunk-y, and Bedlam's is ridiculously extended) In general Bedlam is very like Banyan, and even handles similarly. But slight differences in wheel base and a major change in the braking means its a lot easier to deal with in many ways. Don't tell me no one has noticed how narrowly Banyan passed drops last year--for that to happen in Bedlam would require either the driver not paying attention AT ALL, or the brakes being almost entirely removed.
Even after that there were actual changes. Bedlam's windshield was a pain to form (pointy nose + I wanted to see everything...and no one told me that much screen wasn't necessary),so they took that into consideration when designing Borealis.
And as far as Banyan still being A-team in 2010: that buggy was built to go fast, not really to last. And both of those things are playing out now. One of the things I know the mechanics have been working on is making more reliable (and hopefully freakin fast too) buggies. That makes it easier to carry at least decent speed from year to year without significant rebuilding (in most cases, as far as I know). And from a driver's perspective at least, it's easier to learn to drive well when you don't feel like you have to worry about how your buggy is acting that day. I've never felt unsafe in any of the buggies I've driven, but there have definitely been times when I couldn't tell if the buggy was acting up, my line was actually wonky, or some combination of the two.
So that was really long. Sorry. If I'm super off base on anything at all, feel free to correct me. Or to yell at me for telling super secret secrets that seem obvious enough to me to not think to hide. OR for just not making sense.