Apple succeded in bringing its iPhone developers across to its tablet platform. It forced them to redesign for the larger screen in the process. Those that did were rewarded with higher sales, more reviews and features from Apple. Now it is pretty much a requirement to build an iPad specific app.
Android on the other hand promoted "write once, run everywhere" to support the fact that an android device can be any resolution, any screen size, any PPI. A few tablet apps were released. The problem is that in Android Market all apps are equal, even on tablets.
A developer who spends time optimizing their Android tablet app is rewarded by it getting lost with the flood of other "tablet" apps that are just list views with 90% whitespace because they are scaled from a phone.
Windows 8 will be interesting though. I think a lot will depend on the hardware and the cost. If W8 can produce something akin to the iPad for the same price point it might be able to make a dent in the iPad's market. The real problem for Microsoft though is that W8 works terribly with a mouse and keyboard, from my experience.