Why don’t we eat caviar from other fish?

Answer 1:

Yes, we do. It’s known as fish roe.

Only sturgeon eggs are referred regarded as caviar.

Answer 2:

Sturgeon roe is known as caviar in the commercial world. Other types of fish roe have other names, or are simply referred to as roe if kept in salt, or are referred to by other names if preserved in a different way (for instance bottarga is died mullet roe).