The main reason that strata management companies stick to certain banks, is because those banks are efficiently set up to specialise in the services needed in strata management, e.g. software and such as mentioned by Whale. Also, if the strata management company holds all their trust funds in the one bank, they can usually negotiate good deals for their clients by way of interest rates and fees.
Secret commissions are a big ‘no-no’ if a strata management company wants to keep its license. All commissions must be declared in the Agency Agreement between the strata management company and the owners corporation! No reputable strata management company would want to take the risk!