The naturalistic paradigm, which is essentially a religion with its own fundamentalist true believers, regards the non-existence of the supernatural as one of its axioms. The paradigm is inextricably tied to atheism. One might as well ask, "Why won't mathematicians consider the possibility that triangles have four sides?"
This is why the evidence for Intelligent Design is such a threat. The
evidence on which the proponents of ID focus is precisely the sort of stuff that science has always investigated and analyzed. If the
best inference from that evidence - again, what scientists do all the time - were "Intelligent Design," this would threaten the entire naturalistic, atheistic paradigm. A new paradigm would be required - which, again, has occurred throughout the history of science.
As the proponents of ID point out, the new paradigm would not have to posit the Christian God, or any god, as the Intelligent Designer. Perhaps our reality is a cosmic software program, and the Intelligent Designer is some cosmic version of a 15-year-old whiz kid working on his cosmic science project. Or perhaps beings in another dimension created our reality.
The new paradigm would simply be, "We may never know exactly who or what designed our reality, but the best inference from the evidence is that it was intelligently designed." But even this modest new paradigm would overthrow the old one and open the door to a theistic or even Christian explanation, which is why the proponents of the current atheistic paradigm attempt to ridicule and shout down the evidence for ID rather than confronting it directly. Again, this is what the true believers of a ruling paradigm have always done - read Thomas Kuhn's classic
The Structure of Scientific Revolutions,
https://www.amazon.com/Structure-Scientific-Revolutions-Thomas-Kuhn/dp/0226458083.
The point is not that the evidence for ID has reached the stage that the naturalistic paradigm should be overthrown. It is that the evidence put forth by the proponents of ID is precisely the sort of evidence with which science has always dealt and should be given a fair hearing rather than being ridiculed and shouted down in order to preserve the current paradigm.
Max Planck famously stated, "A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it." I'm not sure exactly how this will play out since the current atheistic paradigm is so entrenched within the educational system and the scientific community, but it will be interesting to see what the landscape looks like in 100 years.