One of the impediments to housing transactions is appraisals. If appraisals (which are backward looking) don't support the offer price of a house, the financing for the house can disappear.
For low down payment deals, this is frustrating, but appropriate. But for deals involving a minimum of 20 percent down, it is hard to see how appraisers have a better sense of value than the potential buyer who is actually putting a lot of money at risk. It may make sense to allow buyers who put 20 percent down, and whose source of funds is well documented, to get a loan even if the appraisal comes in a little low.