First let me state, I have not read the opinion, but I must ask the judge, what trait is it that makes a judge certain that nothing outside the confines of his or he court matters much?
While I'm not an attorney I believe the point is that in any trial the jurors should decide based on the facts submitted at trial. They may not make their own investigations since that might taint their decisions. This is not unique to this case.
"Trait" is really the wrong word. They're called the Federal Rules of Evidence, and those rules determine what a trier of fact (the judge in this instance) can and cannot consider.
The main idea is that it's an adversary system with both sides getting a chance to make their case. The plaintiff's might omit to mention important points for the defense case, but the defense shouldn't, and vice-versa. Both sides get lawyers. Both sides get witnesses.
Now, in some trials, evidence that one side thinks is highly relevant is disallowed and a jury never even learns of its existence. Quite often in criminal prosecutions truly damning evidence is supressed and the perp walks because the prosecution obtained the evidence in some tainted manner. Nothing comparable happened in the Dover trial. Everybody got to make their case. There was no jury. The judge considered everything presented.
I thought conservatives wanted judges to rule on the basis of law and not personal opinions brought in from outside the courtroom.
You're questioning the essence of the trial system. A judicial decision must be based on the record that is presented at the trial. That's the reason we have trials -- to present evidence for the court's consideration. If a decision isn't supported by the record it can be overturned on appeal.