Although water apparently flowed on Mars, it's not known for certain just how long it flowed.  For instance, a catastropic flood of only months' duration, rather than a drainage pattern that lasts for millions of years, can have a huge effect in geomorphology, carving out very long and deep channels.   That happened on earth at the end of the most recent ice age, when melting glaciers formed huge lakes behind them that suddenly came pouring out when the last barriers containing them gave way.  It is at least possible that large quantities of ice buried in the Martian crust melted all at once as the result of a volcanic erruption, with a similar effect.  In a desert environment even a huge lake can evaporate quite rapidly, showing retreating shorelines of salt deposits that were formed over just centuries, not millenia.  Also, it should be remembered that all evidence collected so far indicates that the present very weak magnetic field of Mars was not any stronger billions of years ago - it is not something that happened over eons as the core cooled down slowly.  That means  for most of the time that life on earth consisted of microbes, the surface of Mars has been zapped by radiation that would tend to break down complex molecules and of course kill organisms.  As Elton John and Bernie Taupin reminded us in "Rocket Man", "Mars ain't no place to raise your kids..."

It's too bad that the overall search for understanding of the universe around us and its origin has been so completely dominated in the public mind by only one aspect of that search, namely the quest for extraterrestrial life.  It's hard to fault NASA for jumping on that same bandwagon in order to win support, but putting so many of their eggs in this one basket runs the risk that if the landers do eventually show that life was never present on Mars, then people will decide "Then what's the point?" of the whole space program, and the whole thing will go away.