= new reply since forum marked as read
I think part of the problem is that "finding life" has not been effectively defined as per an experimental suite of instruments, at least to the point where someone has figured out a definitive series of tests which could give a "yes" or "no".
Obviously if something crawls out of a rock or a rover finds a moss-covered outcropping, there's your answer- but since that hasn't happened yet, I think NASA is still living in the shadows of the Viking missions, which DID set out with the goal of "finding life" (or not) and the experiments gave results which have been interpreted differently over the years, originally (wrongly) detecting what they thought was organic activity but was actually most likely due to perchlorates or other reactivity in the martian soil, and then declaring Mars "dead" which may not be true, either.
In fact those Viking results remain ambiguous to this day.
So -speculating, here- NASA does not want to set out "finding life" as a goal unless they have a reasonable degree of certainty that they know how to ask that question and get an answer which is both definitive and will hold up.
But again, sample return- once you get some Mars rocks back here on Earth where they can be examined 5 million ways to Sunday, that's a different story than choosing what equipment to send all the way to the surface of Mars and what experiments to perform once they're there.
Also, if there is flowing water, I would think that a good experiment to be performed there on the surface of Mars would be trying to capture a sample and then just look at it under a real good microscope. 
One problem with that which I can see out the gate, I believe these brief liquid flows tend to happen on the sides of craters, mostly, from what I've seen-- water seeping out from sub surface areas; not exactly an easy spot to park a rover to take a drink.
