Because its evolve or die, if KnC don't move to 20nm and more powerful rigs then they will be left in the dust.
Basic business sense.
Agree, they have no competition on the 28nm node!
If you have any business sense you keep selling what sells until your competition catches up and takes your market share.
The only reason you don't is that this is redundant stock you no longer use! I.E. you are mining on the 20nm node.
Perhaps they are concerned about how close their competition are to launch, and want to be ready to blow them away ASAP. Better to stop what you are doing now and focus your energy on the next evolution.
I don't know, I don't work for them, however they aren't Samsung or someone that has enormous amounts of production capacity, heck they don't have the buying power or manufacturing power to make the components they need by themselves. They actually bought the entire worlds supply of the VRM modules for Batch1 and had to switch (luckily to something more efficient) for Batch2.
I'm not saying that its not impossible for them to keep on selling Jupiter level hardware and continuing to make money, anything is possible if there is a will. Its not even that they don't have the manpower or resources to both sell, build and ship more of the 28nm designs, although tbh their ordering and back end systems and resources are at breaking point as it is from what I have seen/experienced etc.
I think its deeper than that, I think that there are multiple factors, some of which might be listed above, I think its a bit of a balancing act. I think its also about making a 3TH feel affordable while difficulty levels aren't stratospheric. Theres probably even a little bit of techy geekness in there about being the first to 20nm, about creating something better than and before the competition does. It may even be possible that they want to control the hash rate of the network to a certain degree, again making it more attractive for future buyers to be offered something that stands half a chance of making money.
But to be fair I don't think the OP gives a monkeys about KnCs business practice, all they want is a 600Gh/s miner because they missed out and are kicking themselves because of it.