Because if we see our history the colonialism and early rulers had main capital. Like when Mughals came from Turkey as they were the descendants of genkhis khan do they made delhi(Indraprastha) there capital which was developed by them.
Kolkata (Calcutta), Chennai (Madras), Mumbai (Bombay), were the ports where Dutch, French and Britishers made there main ports. Britishers took Dutch port (Chennai) from them through war and Bombay was made by seven islands and was given as dowry by french king. So mostly these four were developed and now there are many metropolitan city. but these four are main due to history of each city.