Okay, this has a lot to do with publishing a viable product being a (pretty much required) business tactic. I'm going to skip any comments about each version of the system or my opinions of the various incarnations of ICE and just explain the broader reasoning: Eventually you get to a point where you've 'done it all' and continuing to publish new works for the same system is going to cause bloat and degrade the quality of the system due to repetitiveness, power escalation, and so on. Really, that's the bottom line in a nutshell regardless of any other reasoning.
The other reasoning? RM (RM1) originally started as an add-on or modification to other systems (obviously aiming at D&D due to customer base size). Eventually RM grew into a system of it's on (essentially RM2), but as a result of its... organic?... growth was not well suited to gamers new to RM in general. Add to that the bloat issue and RMSS was created in order to try and generate new revenue (from past customers buying the new system) and gain new customers (which is pretty much a requirement in the long run). This basically explains the reasoning why RMSS came to be (I think it's fair to say that's an accurate description).
Now, from there we get into a quagmire of legal issues, management issues, opinions and guesses... and finally Tolkien Enterprises brings the hammer down on them (the first incarnation of ICE) like a fly on the wall. R.I.P. the first incarnation of ICE.
'ICE' as we know it was bought by an individual who then licensed 'ICE' products out to a new 'ICE' (Mjolnir LLC). RMFRP was then created, likely partially simply due to the legal changes of how things work and also RMC was eventually created as a way to capture RM2 customers that didn't want to move to RMSS/RMFRP. However, that group lost the license for 'ICE' products for...
reasons... which were then granted to former freelancer(s) who organized a new company (Guild Companion Publications) who had previously done work for the previous incarnations of 'ICE'. Due to a variety of reasons (the bigger problematic legal ones being IP rights) a new RM really needed to be created. That's where we are now.
So... the new RMU is essentially legally necessary (again, due to IP rights issues) and it is trying to convert RM2/RMC and RMSS/RMFRP users both and/or the new 'ICE' is trying to gain new customers again.
I think that about covers it without getting into personal opinions.