E.g. everyone wants to ride towards downtown in the morning, so there are no bikes left elsewhere, and there isn't enough space downtown to hold all the bikes. And only 2/3 of the morning people want to ride back in the evening, because they're doing other things in the evening.
IIRC, some European cities have whole trucks that carry bikes between neighborhoods throughout the day to solve this problem. But that only works if you have big concentrated bike-parking stations. You can't do this if bikes are parked all over the place on the street.
It would be really interesting if this service would pay you to ride bikes in the opposite direction of demand. But I can't imagine that would be economical. There just aren't enough people to bike from downtown to the suburbs in the morning, plus you're going to have to pay for them all to get back downtown afterwards!