Hinduism does not have a start date.
In olden days, there was no religion called 'Hinduism', it was just Veda (vaidika) or Sanatana Dharma, and it was prevalent all over the world.
But only people in Bharata desh followed it strictly. Other tribes who followed it a little bit or not at all were the Mlecchas, Yavanas (Greeks), Turushkas (Turks) etc.
In past few hundred years, Western explorers called people who live across Sindh river (now in Pakistan) as Sindhus, which over time became Hindus.
Let's look at the 2 core concepts in the 2 most prevalent religions today - Surrender in Christianity and Namaz in Islam.
Surrender is equivalent to Sharanagathi in Hinduism. Even etymologically, it sounds the same.
Namaz is equivalent to Namaste in Hinduism. Again etymologically, the word was derived from it.
If you look at ancient texts in any religion, they have words and concepts which ultimately derive from Sanskrit/Vedas.
e.g. The word 'Brahma' is related to word 'Abraham' and Judaism/Islam/Christianity are all Abrahamic religions.
When some Christians say the earth is 6000 years old, they probably unknowingly refer to the fact that the present Yuga - Kali Yuga, started around 5000 years back. Their story of Noah's ark is the same as Manu's during great flood of Matsya avatara.
There are Hindu temples in Burma & Indonesia.
Kandahar in Afghanistan was the Gandhara desh in Mahabharata where Gandhari & Sakuni came from.
Kazhakistan was probably Kaikeya desham because in Ramayana, Bharata's wife was from Russia.
California in USA was probably originally called 'Kapilaranyam' - which was Kapila muni's ashram which Sagara kumaras found when searching for the sacrificial horse. The path they dug is now covered with water, hence it's called Sagar (or sea).
There are many more examples to show Sanatana Dharma was prevalent all over the world. Over time, in Kali Yuga, religion deteriorates, so new religions have to pop up to cater to the needs and characteristics of those who are unable/unwilling to follow tenets of Vedas.
ANSWER - Hinduism/Sanatana Dharma used to be prevalent all over world once. It is not prevalent nowadays, because people's minds have deteriorated and they're unable to follow its rules, while other religions have lesser/easier rules.
Source - http://www.kamakoti.org/hindudharma/part2/chap2.htm