Some 2.1/2.2 apps not working
The "Not compatible with this device" message encompasses more than just the OS version.  The Play Store also checks for other software and hardware components that the app's developer listed as requirements.  These components can range from a minimum screen resolution, to having particular hardware (e.g. Tegra 3 chipset).

You can try searching for a given app on Google Play Store website to get a better idea of the incompatibility.