That's very simple. Because the reward at $0.001 is less than they pay for a captcha solve it is not profitable for them. If we put it back to $0.002 they could make a profit again and they would sign up new botnets of tens of thousands every day.

We would like to do it but if we do it would be exploited. It's better to give everyone at least $0.001 every free roll than nothing.