App incompatibilities can be due to a lot of things in general:
Version of Android (Most tablet apps are optimized for 3.0+)
DPI of device (A tablet app will look better at 190 than 240 on a smaller screen)
Whether you're running a stock ROM like Sense or Touchwiz or CyanogenMod (Some methods of Android security for the market do not work on CyanogenMod or AOKP)
Whether you're running a stock ROM like Sense or Touchwiz or AOSP (Sometimes it' just too hard to get stuff to work on those frameworks because they're too bulky in modifications)
Carrier blocks (T-Mobile app is limited to T-Mobile customers, etc)
The type of device you have (You can't run THD apps if you don't have the Tegra chip without something like Chainfire3D)