Because it is an established principle in law that you don't argue to throw out a precedent like Miller if you don't have to. By showing that pistols are militia weapons and were in common use at the time, the Parker Court completely side-stepped any issues with Miller. The Supreme Court doesn't even have to decide that there is a right to keep and bear arms for self-defense. They only need to agree with Miller that the people, including Heller, have the right to keep and bear arms that have some utility to a militia.
Though you never agreed, many briefs in Heller point out that the Surpeme Court in Miller completely ignored the prosecution's argument that only militia members are protected. Had that really been the case, then the Supreme Court in Miller would never have had a need to address short-barreled shotguns or militia-usefulness. They could have simply stated that the Second Amendment only protects members of a militia.
They weren't asked that question. Why would they answer it?
The question before them was if the tax stamp requirement on a sawed-off shotgun violated the second amendment. THAT was the law that was violated.
If the law stated that only militia members were allowed to transport a sawed-off shotguns interstate, then certainly Mr. Miller's militia status would have been relevant. But anyone was allowed to transport a sawed-off shotgun interstate -- provided they had a tax stamp affixed. You keep making this hypothetical the issue when it wasn't.
"By showing that pistols are militia weapons and were in common use at the time, the Parker Court completely side-stepped any issues with Miller."
Side-stepped? They waded in!
The Parker court examined Miller, came up with a "two-prong test", and faithfully applied it to their case. The U.S. Supreme Court appears to have accepted this two-prong test, and added a third -- lineal decendancy.
"They only need to agree with Miller that the people, including Heller, have the right to keep and bear arms that have some utility to a militia...."
AND .... AND .... are in common civilian use. That excludes a whole bunch of weapons -- .50 bmg rifles, select fire M-16's and M4's, machine guns, etc.