Owing to their funding structure based on deposits by many small and relatively uninformed agents, hence the possibility of a bank run and their involvement in the payment system, commercial banks were regulated relatively heavily.
In contrast, investment banks were regarded to need less close public monitoring and regulation, as they were seen to be less important from an overall financial stability perspective and mostly the domain of sophisticated investors. However, the crisis has shown that all actors in the financial system contributed to the build up of systemic risk.