Question

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

Answer 1

Enzyme and substrate works in a lock and key hypothesis. Like a key fits to it's lock
Enzyme has a 3D shape which is precise. The polypeptide chain of the enzyme molecule folded to form a 'pocket' called an active site.
The active site of an enzyme​ has a distinctive shape and charge that complement to it's substrate.
The shape of substrate must fit the enzyme only the reaction will occur. This explains why enzymes are highly specific.

Answer 2

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.