Digital input are quite simple; but analog inputs are far more complex and take a lot of space on the chip.
i totally agree with chelmi.....
If you need more analog inputs, you can use multiplexers.
But if you are using collision sensors and obstacle detectors, you don't need continuous measurement of the voltage. you just need to know when you are about to hit the obstacle! so generally we use comparators. They give you digital outputs, compatible with the controller boards. so you don't have to worry much!