It's down to map design (bottlenecks) and, as Ryz said, lack of organization. Vehicle rushes, if done right, can work well on Field, but they don't happen very often and fail most of the time/are easy to defend against, as Radeon described.
With the base defence nerf in the last patch, it's easier to sneak into AGT/Obelisk/WF, so these buildings get destroyed more often, which is good. Removing the walls surrounding both Refs could make sneaking and tunnel rushes even more viable, but I'm afraid it would also lead to even more tunnel camping.
The worst thing for me are MRLS/Arties camping the tunnel entrances, I don't get how that is any fun! Other than that, I personally think Field is really not that bad as people make it to be.