I agree with post #3, the religions, like Christianity and Islam, that have majorly spread all over have the component of "Go and tell," a strong desire to spread and share their faith with others.
Globalization and immigration also can cause religion to spread. As more people migrate from India to other countries, they are taking their belief system with them. There is a Hindu temple in my town in Texas.
Almost all religions are defined by the culture and geography from which they arose. Only three religions, Christianity, Buddhism and Islam are missionary religions which have reached out beyond their previous geographic confines. Hinduism is uniquely Indian; there are those all over the world who practice it and follow its precepts; but they are culturally Indian. Admittedly, Hindu belief is somewhat limited in scope; however this can be said of other religions which are not missionary, such as Judaism and Jainism. Until such time as it becomes a missionary religion--which is unlikely--it will continue to be defined by the country of its origin.
I can only suggest that God is everywhere for those who believe in him.For those who do not believe in God, he fails to exist anywhere. Others, who hold to other religious beliefs, may only acknowledge God's existence in limited areas (or not believe in him at all).
Essentially, God can only exist everywhere for those who believe in his power.
If God is everywhere then why did the epics of Hindu deities remain confined to India?
Basically, because the Hindu epics were written by ancient, primitive tribal people who viewed their Gods as local tribal Gods. They are Hindu Gods for Hindu people... The authors were not concerned with people outside of their own extended tribal community. Tribalism is very negative and hostile to outsiders, this is reflected in their religious outlook.