It has to do with the role of banks. Banks start out as the middlemen between industrialists. A business that wants to expand to a new market or start a new branch would take out loans from a bank where the business is expected to pay interest. The problem is if you have 1000+ businesses taking out loans from the same bank, the bank, unlike the 1000 individual businesses, has a much better picture of the financial health and trajectory of each one of those businesses and the industrial sector those businesses belong to. The bank is not just passively earning money through interests but also actively investing money in prospective businesses with the goal of earning dividends. All other things being equal and over time, the bank will outplay those individual businesses in knowing who or when to invest, and they will outcompete industrialists who are only privy to the financial trajectory of their particular industry. You do this enough times with enough M-M' circulation, and banks and the financial sector in general will not only command vast amounts of capital, but also completely dominate over industrial capital. Financial capital will transform the economy away from the direct production of commodities and towards an investing economy or in other words, financialization.