Agree with mikf -and even more, when we speak about TOTL keyboards. These things are meant for professionals (also price wise) and pro's don't really change equipment that often as we would think. And when they do upgrade, they try to stick with the brand they already use -simply because it makes continuing more comfortable. And so, there really must be something exceptional "on the other side of fence", to make brand switch to happen.
As for enthusiastic home/hobby keyboard musicians (willing to spend some cash)... For many here, it's probably the first time they even heard about Ketron. I think, we tend to make a "safe" (future proof) decision: we buy what's proven not to be wrong. And here Yamaha and Korg are well positioned, I think. Especially Yamaha, because here we can also find good entry level keyboards. And as we know, once you start with certain brand, you usually stick with it. Ketron however, has no such offer... and so we come to my first paragraph.
Bogdan