Partly this is because we still need to know about the mineralogy of Mars, there is a ton we don't know. Improving on that knowledge will mean that we are all that much more able to target the regions of Mars that are more likely to harbor or have harbored life. And then we can send a "life detection" class mission.
However, such missions aren't easy. Consider a few of the challenges. In order to determine the composition of a rock you really don't care about the extreme minority constituents of that rock. If you can figure out what the elemental composition of the rock is within, say, 1% that can be a good day. However, if you want to determine whether or not a sample of dirt contains living or formerly living microbes then you are talking about a teeny, tiny fraction of a fraction of a percent of the material. Which means that you don't just need to break down the material into its major parts, you need to figure out how to focus in your studies to just that small bit of biological material. Which could mean extremely high magnification microscopes, for example, though that has a throughput problem. Or it could mean making use of various experiments to prove the existence of running metabolic activities within living organisms, such as using radiologically tagged nutrients, for example. But these sorts of things are pretty much a crap-shoot, and wouldn't help if the biological samples are no longer living.
Also, a NASA life detection mission requires much higher standards of clean-room assembly and pre-launch sterilization, which add expense and complication to the mission.
Keep in mind that for unmanned "life detection" missions a spacecraft would not just be constructed in a clean-room but all of its components (as well as the whole vehicle) would be extensively sterilized (at 112 deg. C for about 30 hours, for example). Additionally, the vehicle would be extensively swabbed throughout assembly to search for any amount of biological contamination.
However, more than likely the focus will not be on an unmanned life detection mission but rather on a sample return mission (which would be optimized to try to find samples containing life) or on a manned mission (which would also likely entail sample return). In either case there would be a considerable amount of research resources available to study any samples of Martian life, should it be found, and bring to bear instruments or tests which would unambiguously show it to be of a different origin to Earth life.
Imagine if you were only allowed to purchase $100 of equipment - it could either be a digital camera that can take pictures of fossils and definitively prove life exists on Mars once and for all, which is less likely than, a piece of equipment that detects the presence of methane in the atmosphere. (more likely but less definitive proof).
The way NASA communicates to the public makes it tough. If a troupe of green martians wearing tophats and kilts started doing the "Lord of the Dance" routine in front of the rover, NASA would announce "We are still awaiting test results, but we believe that the Curiosity rover may have discovered organic materials that resemble felt that resembles something that could potentially be a hat."
More seriously, adding extra components to a mission means many more man-hours designing, testing and vetting the extra functionality, and also adds a boatload of extra weight, which makes the entire thing massively more risky and expensive (especially with the rover's at-the-time-unproven landing mechanism). There's no real need to send everything at once, especially if a lot of what you're sending could be ruled out as entirely unnecessary depending on the results of other tests you're carrying out.