I only make freebies, but the same issues apply as with vendors.  Several features of my texturing don't show properly in D/S, and none of the dynamic clothing works.  It isn't because I don't care, nor do I despise D/S users.  I would be very pleased to have all the features work for D/S users, and I'm always grateful and willing to cooperate with a D/S user who wants to make D/S material files for my models.  Likewise,  you can bet that vendors would be keen to have the D/S market as well as the Poser market.  So why don't my models work (fully) in D/S?

• I don't know how to set up D/S materials.  And I realize that one might quickly say "So, learn D/S!", but understand that I work 56 hours a week, and have only so much play time on the computer.  As it is, I have paid licenses for programs which have a higher priority to be learned.  Since the last Vue update, Vue now refuses to recognize my existing content library on one of my computers, and I still haven't figured out how to fix that.
• The material effects which I use in Poser don't exist in D/S.  Or in earlier versions of Poser, for that matter.  Poser has a more sophisticated and flexible material node system than D/S.  So, do I exploit the new capability so as to make the best model I can, or do I keep everything below the least common denominator?  I choose to exploit the new capability.  I am willing to help someone adapt to their program.
• D/S cloth dynamics require setup in a very expensive proprietary program.  D/S simply can't use the dynamic clothing which I make in Silo or modo.  Poser will "clothify" any geometry mesh you want to use.

     You are right in that there is a "football team fan" sort of blind loyalty, on both sides, but I doubt that vendors are shooting themselves in the foot by intentionally not supporting the other app.  Rather, they are simply limited to being able to provide what they know,  and not so many among us are conversant in both Poser and D/S.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5