Why people can't use only one programming language for all purposes?

Answer 1

The same reason why people can't use just one currency

Answer 2

Why not use a hammer for everything? Because then you end it with a tool that doesn't work properly in a lot of cases. Different languages have to make assumptions for their use environment. Html isn't designed for maths, and a compiled language would not be efficient when describing a Webpage's interface.

Answer 3

As nomeh said the same reason why people can't use one currency,different developers need different requirements,So just one language cant never cover all