It was cheap because of the Fed. When the Fed lowers rate, it has an affect on the entire curve. When the entire curve shifts, it shifts the pricing of all spread products, i.e. mortgages, as well.
But that's the point - the cheap money that the FED was doling out was not the only driver. European markets were delivering historically low rates. The Fed Funds rate was increased not once but
17 times between 2003 and 2006 - and all the while the housing bubble got worse and worse.
Despite regular increases in the Fed Funds rate, the rate on the 10 yr T - Libor - mortgage didn't budge because investment firms didn't need to increase rates to get a larger share; they could instead create derivatives and new money which led to the M3 skyrocketing while the monetary base (the only figure the Fed directly manages) remained steady. By 2006 the yield curve was inverted - and investment banks were STILL lending at the same rate as 2003, or better....Because the money wasn't being made on the interest rate spreads.
The Fed was not the only driver, but it was the main driver IMO. Apart from keeping rates too low for too long, their well-telegraphed stair-step increase in the Funds rate took little risk out of the funding markets because everyone knew what the Fed was doing. Greenspan was trying very hard to avoid a route in the bond markets similar to 1994, but it had the perverse affect of increasing risk. There was also the well established belief that there would always be a Greenspan Put, that Greenspan would bail out the markets when needed. Knowing that there was little risk in the market, banks loaded up on leverage to capture the spread along the curve. So even in 2006 when the curve was inverted, there was a general belief in the markets that the Fed would not allow the market to fail, which lead to money managers duration match risky and riskless assets to capture the spread between different markets.
I put Wall Street as culprit number 2 on my list, but way, way down from the Federal Reserve and how it has conducted monetary policy over the past two decades. You cannot have bubbles without a dramatic increase in the money supply, and the Fed is the primary determinant of the money supply.