This is still in comparison to Android. Why does that matter? 3% or 1.25 million. That is a whole lot of consumers waiting to use their service!
The company might be better off taking the resources that would be used to make an app for another platform and using those resources to continue improving and expanding the ones they have.
I would imagine that the less platforms involved, the more streamlined they can remain. What I mean is this: suppose I have an auto parts store. I stock parts for 2 automakers. There is a third automaker, but only 3% of the vehicles on the road are theirs. I could decide to stock their parts as well, but I would need to expand my inventory by 50%. I would need to expand the size of my warehouse and store by 50%, I would need to train my employees for a 3rd automaker's vehicles, and of course my inventory overhead is increased by 50%. Is it worth all this to increase my sales by 3%? It's probably more profitable to use those resources to improve upon what I offer the 95% I already service.
The fact remains that it happens in mobile. Why, we don't know. That's up to the company. A lot of decisions are made based on ROI, and providing apps for iOS & Android is the decision a lot of them make.