Discussing past :
Bengal was the most important presidency in the British India and Haldia port was one of the most important port in India.
Fort William was most important fort of British which consist of all generated revenue, was present in Bengal.
Governor general of Bengal was the consider as Viceroy of India
By looking each and every side of Bengal, Bengal was most important presidency in political and economic perspective for British Empire.
So what happened, that power was transferred to Mumbai economically and to Delhi politically, and now there is not much importance of Bengal and it's capital Calcutta.
Have you ever thought about it?
We will go with two different ways:
First with political way, today the capital of India is Delhi and most of the political affaires are done in Delhi (which was been done in Bengal in British India)
The reason behind shifting power to Delhi is 'security'.
as we known, Bengal after partition got broken into East Bengal and West Bengal.
and further into West Bengal and East Pakistan.
So after partition the capital Calcutta was very close to East Pakistan and China.
Which was easy for them to attack. If China or East Pakistan invades the India it was easy for them to control Calcutta, that is gaining control over the capital.
So decision was made to shift the political power to Delhi bringing it in center.
Now going in economic way. Today Mumbai is India's economic capital this happen due to various reasons,
Partition of India-Pakistan:
After the partition of India Pakistan. A giant population transfer began immediately after partition, lakhs of Hindu people from East Bengal began to migrate in India and got settle in West Bengal.
Nearly 2.523 million refugees from East Bengal migrated and 2.06 million people got settled in West Bengal.
This leads increase in population density in very short span of time and create crisis in Bengal, like food crisis.
Also due to Hindu-Muslim violence various infrastructure homes were destroyed
Jute mill and jute production stopped because major jute production was done in East Bengal ( current Bangladesh)
This leads to down in economy.
Bangaladesh Liberalisation war 1971:
Also at the time of 1971 Bangladesh Liberalisation war large number of people from East Pakistan ( Bangladesh) migrated India.
Over 10million people migrated from East Pakistan to India and settled in West Bengal
This Increase population which again created crisis in Bengal.
In parallelly Mumbai as very good port started developing with the bullet speed
all other business minded people like Gujrati and Rajastani migrated in Mumbai and started developing Mumbai.
Therefore gradually in 1980's Mumbai overtook all importance of Bengal