Before someone comments that we don't need humans on Mars if robots can do the same cheaper: that's beside the point. I mean, robots are no where near performing on the same level as humans when it comes to ingenuity and ability to come up with and implement ad hoc fixes to problems that no one could even imagine before launch of the mission. But putting that question aside, the problem with robotic missions is that they will never get the same sort of funding as human missions. A human mission automatically has to have a certain size, e.g. has to develop capabilities to land payloads in the ballpark of 10 tons or more on Mars. Once we have this capability, we can easily send lots of robotic and scientific payload along with humans -- it amounts to simply using the same payload delivery system that we are developing for humans anyway.
On the other side, if there is no ambition to fly humans to Mars, then no one will develop these capabilities. There is simply no funding for a system that delivers 10 tons of cargo onto the surface of Mars, unless it can also deliver humans, and bring them back safely. So we cannot send big robotic missions to Mars.
Human missions generate lots of excitement, lots of excitement leads to lots of funding. Robotic missions can never be on par with human missions in terms of how much excitement, and thus funding they can raise.