Well, testability is of course a reason. These raid encounters can be really intricate. I have to test my raids with 11 classes and 34 specs (minus heals). Adding other CRs to it means a multiplication of that testing effort and it would mean I would have to pay for CRs, run them, report bugs to the original developer and wait for a fix to even release my own products. That just does make no logical or financial sense. Tuanha's routines alone are ~200 € if I were to buy them all.
If you don't mind a technical answer - here it goes too: I have yet to find a CR that flawlessly implements the so called "CapabilityFlags". We have ways to toggle off certain features of a combat routine like Interrupting, Pet Use or AOE. But most CRs don't respect these settings even though they should as it is part of developing a CR. Dragon Soul has so much custom logic in it, I had to write my own Mini-combat routine and embed it into the profile just to make it work the way it should. If I turn of AOE on Spine of Deathwing and the CR thinks "Rain of Fire" is a single target attack then I am just not going to bother.
But what I am going to do (sorry for repeating myself) is to develop my own CRs. My profiles will then automatically (and for free) use my CRs and with this I can just get rid of the Singular issue. And if people want to use them with other products, they will also be available on the store for sale. But as I have said before stuff like that takes time and I am a single developer. So for now I would suggest that you run Singular on as many profiles as you can and report every single problem in the Singular support thread to light a fire under their ass so they get that thing in shape. Or you live with it until I am ready to give you my stuff you can just try to find ways to make it work with Singular. On certain gear levels which are available through Tanaan, every class should be able to run 25HC.