The problem was that all banks needed were falling short of capital at that time (due to trading and other impairments).
For every $ of Bank's capital, they can lend up to 12.5x of risky assets. So consider they loss say 10% of their capital during the financial crisis, they need to reduce their loan book by 10% in worst case. Indeed they call loans from their prime customers because they know they have money to repay, else they increase their bad-debts => reduce capital => reduce loan book....
Remember every time there was a crisis, CitiBank was selling some of their prime property holdings. They sold their Wheelock House office (then lease back), then bought it back few years back at big premium.