Why do enzymes generally only bind to one type of substrate?

Explanation:

Enzymes are proteins that increase the reaction rate that occurs within the cells.

enzymes generally only bind to one type of substrate because the active site of the enzymes has a certain shape and as per lock and key model, the enzymes have to be of specific shape the same as the substrate to get fit.  

Enzymes have active sites, where the substrate gets fits when the reaction occurs.It means that enzymes generally only bind to one type of substrate and they are highly specific for their substrate.