Across 70k apps, just finding ones you might want to deal with and investing in dealing with them is a chore for the consumer. Especially with so many derivative/clone apps, the "best" will get some money and the rest basically none. With so little real value add by so many, you might need a good micropayment system. If we can't be bothered to pay more than USD 0.001 per song play to a musical artist, why would we pay more than that for an app use?

Supply/demand. More apps, less revenue per app.