I suppose consumers are looking for regrowth rather than just maintenance. If you look at the Propecia 5 yr study, the drug's results are impressive when set against the constant decline in hair counts suffered by the placebo group. However, your average guy taking Propecia will probably simply assume that his hair loss might have stabilised anyway - if he can't see new hairs then he might conclude, wrongly, that the drug has had no effect. Perhaps that's why GSK planned to give people a 6 month 2.5mg/day loading dose for their Phase III Avodart hair loss trials, simply to maximise short term regrowth.
I think there's also an issue of market awareness, though this doesn't apply in the US and other countries, like New Zealand, where pharmaceutical companies are allowed to advertise directly to consumers. In Europe, this is prohibited, so most balding men - and even most GPs - don't even know Propecia exists.