$1mill limit may be a throwback from Govt guarantee.
Business customers cost money - need to provide cash facilities, creditcards, cheque books etc ideally. If they are just going in/out then $$ often don't stay long, plus difficult to administer online transactions I think because of potentially >1 signatory so risk of legal liability.
These are my guesses only, but basically just too hard.
Happy to be proven wrong by someone that knows what they are talking about.
