Because the long tail of the current product is coming to an end. Only so much income can be gained at the end of any products life cycle. For a company to stay afloat, it must invest in a future product in hopes that it's sales can pay for the R&D invested along with keeping the lights on post launch of the new product.
I'm going to be blunt here. Feel free to call me bad names.
Squad stumbled upon a unique idea. Being the only true game of it's kind, KSP has garnered a high reputation based on it's uniqueness more so than it's quality. Squad has a history of implementing ideas, seemingly without refining them before hand, and than abandoning them whether or not the idea was good to begin with. Career mode is the prime example. A hodge podge of unrelated/unconnected ideas thrown together for the sake of added gameplay. Each piece nearly forgotten and untouched since introduction. The UI has always left something to be desired. When gizmos were added, they slapped on place holder UI buttons instead of integrating them. To this day, those gizmo buttons float randomly in the VAB UI. The PAW menus have always had that "stock game engine" UI feel. Instead of accepting that Unity's wheel code would not work for their game and programming their own, they slapped on bandaid after bandaid, making Kerbals phase through wheels and adding UI so the player can "fix" the wheels in game. The first 6 years of KSP introduced artist after artist with an obvious lack of direction and guidance. To this day, it's a blessing from the Kraken if all the whites on a ship match. I could write an essay, but I'm just a handyman. So what do I know.
It is more than possible that Take Two decided Squad wasn't the development studio they wanted for KSP moving forward. Maybe I've just been disillusioned after 8+ years of seeing squandered potential on such an amazing concept. Maybe I'm just a grumpy middle-aged man.
As with every sequel. I don't know how this is a surprise to anyone.