I know what you mean, it is getting more difficult to find the right mixes. I remember when Restek, Ultra, and AccuStandard offered mixes for just about every special list you could think of. Not it is a Mega Mix that is missing just a few needed for a certain regulation list but to get the rest you pretty much have to order the full Appendix 9 list which for semivolatiles is about 175 analytes in 5 -7 extra mixes.
Most of it is because of compatibility between individual analytes, but also the standard companies are no longer small companies driven primarily by offering as much specialized products as possible to satisfy the broadest range of client to strictly profit driven large companies who are now big enough to offer a few products that cover their largest customers.
I remember when the Restek catalog had pages and pages of individual analyte solutions but those have been reduced down quite a bit, and now most are in multi component mixes. The others did it even before Restek did.
The other thing that makes it difficult for the standard companies to offer what we need is each state and even some local jurisdictions are making up their own list that adds one or two analytes extra, or subtracts a few, or has one or two with ridiculously low detection limits compared to everything else in the mix that you need a special mix just to calibrate for that one analyte.(like dropping benzo(a)pyrene 10X lower than the rest of the PAH analytes.)