Someone will be able to explain this better than I can right now but the short and dirty version is that actual banks in the US operate with authority given by the federal government. This relationship grants them perks like being able to operate across states lines, etc., but also FDIC protection which insures the deposits they hold for customers. Since marijuana is illegal at the federal level, banks cannot have customers who are in essence violating federal law. Allowing dispensaries to be customers opens the bank up to a level of risk they are not willing to bear. My comment referencing being "federally insured" is in reference to FDIC, which is the easiest way to tell whether or not a bank is regulated at the federal level. Many credit unions are in fact not "banks" in the traditional sense as they are not federally regulated and do not offer FDIC insured deposits (although IIRC there is another form of deposit insurance that is not federally backed). So again, the short version is your payment path in the marijuana industry cannot touch any corp that is federally regulated.