Most probably the VSL people see it as redundant. In todays orchestras the clarinet in B-flat is the standard instrument. The players tend to switch to the clarinet in A when they play passages with many sharps, simply because it is easier to transpose that way. With a virtual instrument there is no need to transpose - therefor you don't really need a clarinet in A if you have one in B-flat already.
The clarinet in E-flat is shorter than the clarinets in B-flat and A and has a different timbre. It's a different instrument really, while the other two can be seen as the same instrument simply with different tunings.
That being said I would like to see a clarinet in A from VSL. Eventhough it is not a very pronounced difference the clarinet in A has a slightly mellower and softer timbre than it's higher pitched brother. That's why it has been used quite often in chamber music.