Why are Sindhis, Gujaratis and Marwaris so good at business?

Because that's what they hear, day in & out.
Not sure what you guys talk about in family gatherings, but in our house it's mostly business. Why somebody failed, how somebody grew, what xyz is selling, how, etc etc.

Plus the connections, most of my relatives do business, it's a broad network.. And it does helps.

Not all are good, also there are other communities people as well who are very good with businesses. There is no point in generalizing