1) Mars captures the public's imagination far more than the Moon because:

a) There is the possibility of life there.
b) There is much more colonization potential for Mars.
c) It has been geologically active and has much more potential for helping us understand Earth better through comparative planetology.

2) Unlike flags and footprints Apollo missions, US mission plans give plenty of time for real detailed research on Mars.

3) The combination of research time and more avenues of research means Mars mission results are more likely to be of interest to the public.

4) A huge portion of the cost of a mission to Mars is development cost. With follow-up missions costing less than a years worth of Shuttle flights and occuring only every too years why would you assume public support lower than for the shuttle? They would be far less routine and far more inspiring.

I sure don't claim we are garaunteed follow-up missions, but I think Mars stands a better shot at that than other proposals I've seen based on long term potential and public interest.

Regards,
Mark