The main reason I'm skeptical of life on Mars even in the past: Life is tenacious, in all its forms.

Curiosity rover: NASA finds new evidence life may have existed on Mars

Finding evidence of past riverbeds, which means a component of life as we know it on Earth was present, water, is nice, but not anything to get excited about. If we find evidence of something that was the cause of a planet wide extinction level event, then I'll stop being so skeptical. On our own planet, 99% of species went extinct in less than a million years 250 million years ago, yet here we are still talking about it. Life is tenacious.