We do not know, for a fact, that the traces found in the mars meteorite are actually fossils. If they are, however, it is not surprising that Viking didn't find anything, especially if any indigenous martian life vanished over 3 billion years ago as the fossils might suggest. If any life still exists on Mars, it is probably deep underground, and the Viking landing site is probably not the place to go. A much better spot is the South Polar Cap which we know contains lots of frozen water.