I think part of the reason that walkthrough tortoises are not any more common is simply that....tortoises don't do much.
If you think about the animals which are commonly kept in walktrhough enclosures - think lemurs, monkeys, wallabies, tropical birds... they all have something in common - they are very active at daylight hours. They are always up to something.
By all practical purposes, a walkthrough tortoise enclosure is not in the least bit impossible. It's just that it wouldn't be very engaging for visitors methinks.... unless they really like tortoises.
And there's also the question of keeping the tortoises warm during winter.