I think it is nto a lack of money but a lack of buyers in the secondary markets.
BINGO, investors are finally scrutinizing all the golden mortgages and find that they are merely gilded lead!
Same thing.
It takes Buyers in order to provide liquidity. Lending institutions have to sell old debt in order to get the money to make new loans.
With no buyers, there is no money to buy the old loans...which means there is no money to make new loans.
That's how you get a credit crunch.