This is speculative. I don't know whether it works empirically. Or, I should say, I don't know how much it works empirically. The effect I am talking about here might be large, and explain almost everything. Or it might be small, and explain almost nothing. I'm just throwing it out there.
Legally required reserves, if those reserves pay low or no interest, are a tax on banks. For a 10% required reserve ratio, where required reserves pay 0% interest, for every $100 in deposits a bank issues, it must give an interest-free loan of $10 to the government. If market rates of interest are (say) 5%, that is equivalent to a 0.5% annual tax on deposits held at banks.
The main reason that Canada got rid of required reserves 20 years ago was precisely to eliminate this tax on banks, and to 'level the playing field' between banks and other financial intermediaries. (And because the Bank of Canada figured it did not need required reserves to help it control monetary policy.) The Bank of Canada also pays interest on the very small amounts of reserves the banks do choose to hold, and that interest rate is only 0.25% below the target for the overnight rate of interest.
If you put a tax on apples, and not on pears, and if apples and pears are substitutes in both supply and demand, the market will respond by producing fewer apples and more pears. The pear industry will expand to avoid the tax on apples. We will see more pear trees, and fewer apple trees.