I suspect that some of it has to do with 29er's being generally fast cross country bikes, which suits the role of a hardtail. There are certainly plenty of 29er hardtail eMTB's available, and up until this thread I hadn't ever given this any consideration.
Edit... The diagram below shows the comparison between the various sizes.
I've test ridden a number of haibikes with both Yamaha and Bosch.. The Bosch is a much more enjoyable ride, even thought the specs on the Yamaha look better on paper.. Experience is the true test.
The one I liked the best was the 2017 Full Seven S 7.0 .. 28 mph top very smooth and weighs less than the competition.. .I think it's a 650 tire but they have one in a 29er.. 650 is much better imo, but to each their own!