I think the main reason some vendors stopped creating for Poser after moving on to DS is that they probably stopped buying Poser altogether. They would have to have the latest version and provide support if they wanted to continue creating for Poser. Let's face it, DAZ sales probably are much greater than the vendor would get at other stores so they seem to be satisfied (stonemason pretty much said so in a thread where he explained why he stopped creating for Poser).
Don't forget DS is free so many new vendors came into the 3d market via DS and didn't even bother with Poser as the Pro version is quite pricey and they would have to learn a whole new program.

I highly doubt it's software politics because money talks and if a vendor is not making enough supporting just DS then they would create for the Poser market too, I don't think long-time vendors are that naïve as to be coerced to settle for less income.