Whole bunch of incentives.
As is noted below, usually there's an upfront option fee. It's almost never 20%. It's often closer to 3%-5%. Still, that can be a chunk of money. On a $400,000 home, you could be talking about $10,000-$15,000. It certainly could be less (or more). But even if it's only 1%, that's still $4,000.
Second, usually on a lease-option the amount the tenant-buyer is paying is slightly above market rent, with the money (or more) being credited to the purchase price. Let's say market rent is $1,500. On a lease option, the tenant-buyer typically would pay a bit more (say $1,700) with maybe $400 being credited to the purchase price. If the buyer doesn't buy, then he/she has paid an extra $200 a month. On a 2-year lease-option, that'd be $4,800. Even if the TB does buy, it's an additional $200 a month. That's a nice incentive for the seller.
Third, if the seller is having a difficult time selling the property, at least he's found someone who is interested in buying. No guarantee of a purchase, but at least there's some possibility. And in the meantime, he's got monthly cash flow.
Fourth, a properly structured lease-option makes the tenant-buyer responsible for most of the repairs and upkeep. That can reduce the maintenance headaches for the seller.
Fifth (and check with an accountant on this one), option fees aren't taxable until the option is exercised or it expires. So let's suppose someone moves in, puts up a $6,000 option fee and lease-options with $200 a month representing the option fee. And let's suppose the tenant-buyer stays there the full two years. At the end of the two years (not this year, not next year) the taxes would be due on the $6,000 plus the $4,800. Let's take a worst-case scenario: Person lease-options the property December 1 and then defaults and moves out February 1. Taxes on the $6,000 option fee wouldn't be due for the 2009 taxes; they'd be due when the TB defaulted and the option ended in 2010.
There are a whole bunch of other reasons a seller would agree to a lease option. Properly structured, they can be win-wins for both parties.
Hope that helps.