Because this would vaporize squirrels.
And this one won’t take down deer.
And this one is convenient to carry.
And this one fits in my pocket.
These ones are great for killing fascists.
And this one is great for killing commies. And deer. And zombies.
And this one is good for playing cowboy.
This one is good for clearing bunkers and trenches. And killing zombies. And clearing zombies out of bunkers and trenches.
Basically, the same reason I have more than one screwdriver, more than one hammer, more than one wrench, etc. Different guns do different jobs.