A couple of reasons. The first is that to make a mission to Mars cost effective, we have to wait about 18 months for Earth and Mars to line up in the right positions, otherwise it costs too much in rocket fuel to be worthwhile. The second is that the missions lag behind the discoveries - it's much easier to analyze pictures and spectographs than it is to design a mission, secure funding for it, and then do it. Sending a landing craft to Mars is not very easy. We've done it successfully a few times, but we've failed quite a few times as well. A lot of these discoveries happen in places that it isn't trivial to land a rover. We put them in flat deserts because it's safer to and there is still much to learn there, but liquid water doesn't necessarily get found in places rovers can go. I imagine we're much more cautious with rovers on Mars than we are on say the moon. It's very costly to make a mistake and lose a rover on Mars.