Explain the formula of convolution and convolution theorem
Because the front part of the input hits the filter first.

The graphical diagrams shown in most text books with an exponentially decaying signal multiplied by a step function show this directly. 

While Causal response is not required for valid convolution, it is essential that the filter output responds and evolves from the head of the input. 

Convolution also commutes, so the head of the impulse function also hits the signal first.  

Correlation overlays the pair 
 of functions which why it is typically used in measurements associated with similarly measures.