You answered your own question, really!
Just because they are both Paphiopedilums, intersectional hybrids are not really as compatible as you seem to think that they are.
Sanderianum produces hybrids with a great deal of form issues. Again, a long petaled Paph. Crossed on to another parent doesn’t mean all of the children will be beautiful!
Plus what Rob says, I have heard myself a few times. Very low seed counts producing fewer seedlings. Instead of raising a few hundred hundred seedlings, it could be a couple of dozen. And I think or I suspect that intersectional hybrids might be slow growers, very shy bloomers. So if you have a couple that bloom often, treasure them.
AND if a hybridizer gets a few seedlings, they may want to grow them all to maturity before parting with a few. Not everyone is breeding Paphs as a way to instant wealth! In fact wealth might not enter into the equation at all.
I guess to a large degree, we have to learn to be happy with what comes down the road. We as hobby growers do not control the seedling market, the genetics might have the real control.
About 30 years ago, I picked up a compot of Del Rosi. 9 seedlings. All matured. All grew very slowly. A few died. Not a single one ever flowered for me!! That says a lot right there.