Because they fear regulators.
Cash is actually pretty hard to move around. There is only so much you can move in a suitcase before you need large transport which can be stopped at borders.
Electronic transactions enable meaningful amounts, lorry loads, of money to be moved across borders every easily.
Once banks can't trace the movement of electronic funds and they can't rely on the connectors that maintains those funds flowing from keeping regulatory compliance, then they figure its a risk to be linked to Bitcoin in anyway.
A bitcoin wallet is bank account. Banks aren't holding back bitcoin, its the supply chain that still only uses fiat that is holding back bitcoin.
If you want to help, ask to be paid in Bitcoin, in work or in business.