Part of this is that it's expensive to do experiments on Mars, and space is limited. Each experiment you choose to do excludes you from doing others, so NASA has evaluated the evidence and doesn't think its the most efficient thing to pursue right now. Levin is trying to convince folks that the evidence is sufficient to warrant pursuing further experiments that would clarify what is going on here.
Consider for instance how many US citizens are still religious today, and largely govern their lives according to religious beliefs. Consider how the current president has deliberately weakened the separation of church and state by making it legal for churches to be political - pastors of tax-exempt churches can now outright advise congregation on who to vote for.
Discovery of life elsewhere threatens the foundation of a lot of religious dogma.
An irrefutable discovery of complex alien life on another planet would, in my opinion, undoubtedly accelerate that rate of diminishment.
In the mean time, many people use the absence of any such discovery, as proof that earth is inexplicably unique so some magic creator deliberately blessed us to be here on this sacred planet.