In his part of the majority opinion in United States v. Booker, Justice Stevens had what I think is the most honest explanation of the origins of the Blakely revolution. As sentencing guidelines schemes became increasingly popular in the 1980s and 1990s,

the Court was faced with the issue of preserving [the] ancient guarantee [of the Sixth Amendment] under a new set of circumstances. The new sentencing practice forced the Court to address the question how the right of jury trial could be preserved, in a meaningful way guaranteeing that the jury would still stand between the individual and the power of the government under the new sentencing regime. And it is the new circumstances, not a tradition or practice that the new circumstances have superseded, that have led us to the answer first considered in Jones and developed in Apprendi and subsequent cases culminating with this one. It is an answer not motivated by Sixth Amendment formalism but by the need to preserve Sixth Amendment substance.

  In the view of Blakely proponents, times had changed, and the Court needed to rethink Sixth Amendment rules "to preserve Sixth Amendment substance" in light of new sentencing guideline regimes.

  The obvious question is, how could Justice Scalia and Justice Thomas join in this example of what might plausibly be called "living constitutionalism"? My speculation is that there are two reasons.

  First, Justices Scalia and Thomas are open to creating new constitutional rules when they think that something new is needed to restore the function of an old doctrine. If changing technology or practice threaten the function of the old rule, Thomas and Scalia are willing to create new ones. An interesting example of this is Kyllo v. United States, in which Justice Scalia's opinion, with Justice Thomas on board, created a new Fourth Amendment rule to regulate thermal imaging devices. Changing technology threatened to eliminate the Fourth Amendment's traditional protection of the home, and so Scalia created a new rule to try to restore old protections.

  Second, Justices Scalia and Thomas much prefer rules to standards. That is, they like clear legal rules knowable ex ante instead of mushy balancing tests applied ex post. Apprendi/Blakely opponents never came up with a rule to protect the Sixth Amendment jury trial guarantee, while Apprendi/Blakely proponents did. Faced with a choice between a rule and a mushy balancing test, Justices Scalia and Thomas naturally gravitated to the new rule adopted by the Court in Blakely.

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