Disclaimer : Academic post, not meant to be normative, the word "Bania" is used bona-fide.
One of the undeniable realities of Indian history has been the caste system. A little thought on the same(induced by formal history reading) would lead one to conclude, that this social stratification had not been uniform throughout the country. The caste divisions as seen in Uttar Pradesh are different from those in Tamil Nadu.
One of the more peculiar things regarding this that I realised in hindsight was the absense of the trading stratum within the Maharashtrian caste heirarchy. There are Brahmans, there are Kshatriyas and Shudras and there is a "warrior+peasentry" mix in the Marathas, there are Dalits, but no Vanika. Such an absense of the Vanika is not seen starkly in any other place, which have their fair share. I think I can safely argue that on an average, having business sense(or business shrewdness) has never been the common man Maharashtrian's strength. This is also shown by the absense of any big Maharashtria names in the field of business, (specifically as business owners).
I think that one of the outcomes of this was that trading people from other places, came in and filled this vacancy. As much as different strata of the society have a varying degree of influence on the society in general, it can be argued that the "having" of economic clout goes a long way in determining the political (and other) clouts. Consequentially there is absence of Marathi clout specifically on the economic front in Mumbai. The popularity of people like MNS and SS could be attributed to this resentment.
Having said all this, it is possible that my observation may be incomplete and hence flawed.