I believe Remitary got tired of the game, this is proof with the 4th floor as there was nothing ever there. Visually there were things, but 4th floor was a dead floor. Though I am sure some could argue 'that it was basically end game,' floor. Because after that you had the final showdown, but you were never in any rush to do it at the same token.
I sort of get where you're coming from, because the last floor does have relatively less content when you consider it doesn't have an entire unique mini-game like other floors. But also keep in mind, every time another income mini-game gets introduced the entire economy and balance of the game has to factor in those changes, if I was Remtairy in that situation I'd also be sick-to-death of doing economy overhauls for new mechanics, and not just that, but if you work on something long enough, the fatigue can actually blind you to critical errors in balancing, especially for a game made to accommodate different builds.
I think it's fair to get a certain distance in a small team, and decide you've had enough of development with this idea, and tie a bow or knot on it and move on to the next project. I think I mentioned earlier on in the thread quite some time ago, that I'm also excited to see what their next thing is, because they've mentioned wanting to do different things and never stay stagnant. That mentality that maybe has them bored of working on KP, might be the same kind of energy that made KP innovative to begin with. I'm sure there are talented modders who might have ideas to fill in the blanks or try overhaul the experience, if not, there's always the next project from Rem.