Depending on how it was written, porting a DOS application to Windows may be non-trivial. It was probably deemed not having a high enough ROI (return of investment), when considering the expected revenue they would get from all the Designer/Forever/Collector packs.
If you are talking about making a Windows port that is completely reprogrammed from scratch, to be identical to the PSX version - that requires even more work, and probably a full team working on it. Kinda hard to justify, compared to working on a proper sequel.
Besides, early Windows applications are not very compatible with modern versions. It is usually easier to get Rayman 1 to run reliably under DOSBox, than it is to get Rayman 2 to run under XP or later (although GOG version of Rayman 2 made the compatibility better).