I suspect that's because it's cheaper to do blockwork and timber than cavity brickwork.
I'd be surprised if it's an "appearance" issue because I don't think there's a mechanism to insist on that (and who would think it looked right anyway?)
If it was to improve the view FROM the henge while killing the view OF the henge I'd be more than surprised I'd be ruddy outraged. Avebury's the bleeding World Heritage Site not bleedin' Wiltshire.....