Why is that?
Why is impossible for the President to put up anybody other than a squishy moderate?
Say the President puts up a principled conservative. A clone of Robert Bork, (or maybe Bork himself!). The Democrats and these "moderate" Republicans will no doubt consider this an "Extreme Circumstance" and will we are back to square zero. But this time, the coalition to preserve the unprecedented filibuster has already been forged. There is no way these "moderates" will vote for cloture or to change the rule. Not with the Supreme Court on the line.
In order to be approved, a Supreme Court nominee will have to be acceptable to the Republican Majority and five Democrats. This eliminates anybody who supports limits on abortion, support for the 2nd Amendment, or a reasonable reading of the Commerce Clause, just to name a few issues.