Weight. I've ordered a T28 and did have the dilemma of whether or not this shall give me enough payload. I didn't want to pay the extra for a T30 however as I'm going to upgrade the suspension anyway and I believe this is where the additional cost goes so would be a waste of money.
Assuming a 2t mass in service weight this steel leaves 800kg of load. That's 10 average sized geezers. Allow 5 passengers including driver, then I've got another 3 geezers worth of payload for the rear seat, bikes and other stuff.