It's interesting how the Chinese diaspora was able to drive a large amount of China's economic growth, but the same is not true for the Indian diaspora.
I'd argue that theres a lot of factors at play. Personally, my parents belong to said diaspora and I spent a long time in China. Ive asked myself the same question. First of all India started out a lot poorer than China post independence. The work needed to bring india up to speed is a lot more. The second issue is one of ethnic identity. China has been working on a predomonantly han based identity for 2 millenia. Indians dont quite have that. Each Indian ethnicity has their own set of values. Also a lot of educated Indians are more familiar with English than there own mother tongues making themselves feel more at home overseas than back home. Then there is the problem of India's financial policy - transacting money is a pain in the bottom for people with money overseas. In recent years things are changing. Younger indians do have a stronger sense of national identity and thanks to technology payments are somewhat easier. The indian government however still does not recognize or know how to exploit this diaspora.
I don't think a common national identity is a big factor. From what I've read about economic development during China's reform-and-opening-up period, private investment from overseas usually piggy-backed on existing trust networks between family and friends. E.g. a Taiwanese investor would support his second cousin's business in Xiamen right across the strait, not some random stranger whose trustworthiness is completely unknown.
So the Indian diaspora split into dozens of smaller groups could provide the same level of support as a hypothetical unified diaspora, each group contributing to their ancestral region's development, just like the Chinese diaspora.
Yes, taiwan, hong kong and singapore have played a major role in China's development. And yes, trust networks are important. One thing to note is that despjte the divergence of the diaspora culturally linguistically mandarin is commonly spoken among educated classes.
Ive observed similar behaviour among tamilians overseas where an enterprising cousin builds their start up around their home village in tamil nadu. However, I think this is where the issue of class comes in. Annecdoteally, a lot of the indian diaspora come from already wealthy backgrounds which do not see the need to help each other.