Why indian ocean is called so?

It's purely down to politics and history. It was named when Britain were building an empire that included India. The Dutch had a very large navy at the time and if their interests had been different, it'd probably be called the Oostelijk Ocean. Likewise, if the British had focussed on Africa, Australia or Antarctica before India, it'd probably be the African, Australian or Antarctic Ocean.

In short, it was named solely because of what was happening at the time it was named.