Just my two cents on this. I’ll second that while made for Serato (i.e. that is what you get with the controller, there is a manufacturer(s)-supported mapping and such), if you have the midi command listing (available) and/or as a software manufacturer want to invest time/money/resources into making a mapping for your software, you could.
However, RekordBox DJ (not talking about the collection management bit) was probably made to compete with Serato, but preferably with Pioneer gear (point in case a whole set of controllers identical to their predecessors, just RB-ready - and unable to map with any software!). Also it was likely made to prevent all those regular RekordBox with CDJ users that also wanted to use controllers to move to other DJ software. This way they could keep their existing installed base and even lock them harder into their eco-system.
As new software, they never put much emphasis on midi-mapping options for non-Pioneer gear, as there was much other stuff to implement/improve first.
So, generally speaking, there are manufacturers of both hardware and DJ software (NI also comes to mind) that are invested in their own eco-systems as opposed to DJ software manufacturers like VDJ, DJay Pro, Mixvibes, Serato and others that don’t make their own software and thus need to support as wide a range of controllers as they can to build a large enough market.
It is always interesting to see how these things play out.