Oxygen was excluded from the experiment on purpose, since it would destroy anything that formed. Other than that, they had no idea what the 'earth primitive atmosphere' (is what they called it if Im not mistaking) could have been like. The filter in the cycle was added because if the protiens were sent through the spark again, it would destroy it.

Even if they did make life, although the experiment was not even close to what a natural environment would have been like, it would prove only one thing. That it takes intelligence to create life.