The more compact nature of the layout means you pretty much need more than 2 rows, or fingering some things seems to become very difficult. It feels like your fingers get tangled more easily if you stick to two rows (which you do at first, out of habit). I don't think it adds much to the cost (the different places to play the same note are all on the same physical lever), but it certainly makes it look intimidating, and the "playing by touch" issue is unsolved AFAIK.

I think the physical size of pianos is why the first decent design "won"; you don't take your own piano to where you're playing, you play whatever's there. In the electronic age there's a chance for Janko to take off, as that's not universally true any more, but I'm not holding my breath.