Conditions for realizing a filter
The simplest reason is that all realizable filters need to be causal. 

A physically realizable filter can't predict the future. If it were to have any type of response BEFORE the input actually happens, it would need to know that the input will occur before it does. When we're analyzing a filter as a system, we usually consider the input to come in the moment of t=0, so for any t&lt;0, the filter doesn't know that the input is coming and can't produce output.

Imagine that you're driving a car and then suddenly you see a car in front of you in your lane moving in opposite direction. You move your car to a side of the road and avoid the car going straight towards you. This would be an example of a "realizable filter". You couldn't have known that there is a car coming towards you until you saw it, so it was impossible to respond to the other car until the moment you detected its presence. 

IIR filters and other types of non-causal systems can be used for batch processing of data that has been already gathered, so that we already have the "future" recorded somewhere. Even in that case, we need to properly initialize starting values for the filter or to design the system in such way that the first few values don't make a major impact on the results.