Fear.

They have to prove you wrong, because if deists are right then...OMG...the atheist might be going to Hell.

It never occurs to the atheist that at some point in your life you did listen to that reason,...you considered it and other ideas...and then...you came to a different conclusion.

You also have a culture issue of prostylitizing.

Everyone must believe the same thing so we are feel comfortable in our rightness.