why don't we can green peppers like we can tomatoes?

Answer 1

Canned green peppers are available in most supermarkets, usually in the ethnic food aisle as part of the Mexican section. Those use special peppers that stand up to canning much better than ordinary bell peppers do though.

As others have told you, peppers are not particularly acidic, so if you are looking at canning them at home you are limited to pickling them unless you want to purchase a pressure canner (this is specifically not the same thing as an ordinary household pressure cooker!).

Answer 2

Some people do. You would need to pressure can them, though, since they don't have the acid of tomatoes.

Answer 3

Not enough acid.