Probably the same reason the UK license-fee-funded BBC initially supported iPlayer on Windows only - it's the most popular tablet platform (or so I believe). The Kindle\Android flavours are apparently on their way, let's give them a chance eh.
However there are totally open formats which you could use on all platforms...
I'm not sure the popularity of the iPad is actually as high as everyone thinks. I think that's the Apple marketing reality distortion field. I see WAY more Kindles and Android phones than I see iPhones and iPads on trains in and out of London where I live.
The iPad is by far the most popular tablet and still makes up over 50% of the market. The iPhone is a vary popular phone, but there are far more options for phones out there than tablets.
Define "competitor" and "popularity". It's all damn lies and statistics.
My old Windows tablet could be a competitor. Dell do a tablet Windows machine. Motorola do a hybrid device. Is an ebook reader a tablet? Is an iPod touch a tablet?
Perhaps people with iPads hammer web sites? Perhaps people without iPads don't? Perhaps some mobile browsers identify themselves as desktop ones?
There is no defined taxonomy to classify things in either.
