It's a new truck because it's lacking the tarp canopy, and the props. This is how the new trucks have been coming in and they've been fitting the props onto the trucks either right before testing on the ride path, or after.It might have been broken down. It didn't look like a new vehicle to me, but even a new one can breakdown. Otherwise, I am sure that Disney has many non-public access roads all over the place where they might have driven it there. There are many ways to transport vehicles, that is just the one that they decided to use that day. I don't think that Disney wants the public to see a vehicle on a tow truck which would denote a broken status. On a flatbed, it could be anything. I'm just throwing out guesses here.
What I did see during the film is that the driver was very aware that, at best, it was going to be close to not going under the bridge. He slowed to almost a stop before it actually hit anything. He took a gamble and lost big time.
They're towing it, because like @jakeman said, there are governors on these vehicles, and if not activated by a puck, they will only go 8 mph. At BEST, 20-25.
The vehicles come to Disney with the governors fitted and I doubt they're going to adjust that just to take the trucks between central and Dak.