Most of the time, the reason for this is one of three things:

1. They do not do what it takes to qualify for a mortgage.  Credit repair takes time.  This means that you need to start working on it as soon as you sign a contract.  You can either hire a credit restoration service or repair your own credit.
2. They fail to negotiate the contract so that they have the time they need to repair their credit or document their income.  Most rent to own contracts are for a year or less.  The reality is that it is going to take most people who do not qualify for a mortgage at least two years to get their affairs in order.  And, if you have had a recent foreclosure, you should count on it taking you at least three years.
3. The purchase price is set to high.  There are a group of investors called lease option investors.  They buy homes that they plan to hold long term, or they negotiate long term lease options with distressed sellers.  They then seek out people with bad credit and offer these homes to them with terms that will never allow people to actually purchase them.  So, despite the fact that the tenant has improved their credit and can get a mortgage, no lender would lend to them because the home is not worth the asking price.

By avoiding these three things, you can greatly increase the odds of your success.