I suspect its because while most of the current hardware is similar its not exactly the same, iPad, Android (Tegra 2) and the Playbook while very similar on paper in regards to CPU's and RAM have totally different graphics processing units which I suspect makes it harder to get the most out of the tech.
Also compared to say PC tech, the CPU's and GPU's are massively underpowered in order to preserve battery life and keep heat emissions down, so a Dual Core 1Ghz tablet CPU isn't the same as a 1GHz Dual Core Intel PC CPU.