You're talking about being entrusted with half a million dollars of someone else's money. That's a huge bet to make on you; it goes both ways. If you can't afford the down payment, then you can't afford to live in that neighborhood.
30% isn't an unusual downpayment, especially if you don't have perfect credit. With a smaller downpayment, the bank is in a position where you could default on the mortgage, they repossess the house, and because it's depreciated in value at all, get less money when they sell it than they paid the person who sold the house to "you" (to the bank, really) after foreclosure and closing costs.