The analogue of the Riemann hypothesis for curves over finite fields has been shown by André Weil (see also Roadmap to Riemann hypothesis for curves over finite fields) and further deep results (Weil conjectures) are known to hold for the zeta function in this case.
What I want to know is: what makes proving the Riemann hypothesis so much harder than its analogue for curves over finite fields? What makes it hard (probably impossible) to generalize the ideas that were used in the finite field case to a proof of the Riemann hypthesis?