If you could make 1.5% off other people's money how much would you "invest"?
They got too much in deposits very quickly and could not originate loans at the same speed. If they kept the money uninvested their operating costs would have eaten up their principal (even if they had to pay 0% in interest to their customers)
but operating costs don't rise linearly with deposits (i imagine it rises sub-linearly, at most), esp. if they can choose to pay 0% interest for deposits.
It really is just a matter of money making. They didn't want to accept the low yield, but certain safety, and accepted a higher yield, but with risk. It caught up to them.
1% of 100 billion dollars is a billion dollars. They would rather have 1.5 billion than .5 billion.