A BILLION iOS devices is quite a “niche market” ... :-) ...
Most likely because the iOS devices cost hundreds, as opposed to the cost of a decent computer from them, which is over 1,000 to 2,500 at the highest, and it’s really impractical to build a hackintosh, a cheap way to build, as well, plenty of compatibility issues. It was a good venture that they engaged in starting in 2007, and it has paid off to sell phones and ipods, and not just costly computers. It’s part of what got them out of the ditch that earlier management got them into.
People who think AAPL plays in niche markets need to rethink that position and don’t really understand marketing strategy.