The point of any criminal justice system must be to obtain a just result. If the procedural rules designed to ensure justice are "inconvenient" to a local or state official's political ambitions to be "tough on crime," so be it; that's the price of the system. Further, given the huge delays that everybody already knows exist in the capital punishment system, there is no justice-based motivation to attempt to evade those rules and just add more possibilities for delay. Of course, that's not how all too many prosecutors see it: They see the system as one intended to result in convictions, not in justice, and resent anything that gets in their way.
The corollary is a simple one. Prosecutors who do not follow Vienna Convention requirements—they really aren't that hard, and require only some really routine cross-checking—must not have much confidence in their cases or their skills. If they did, they wouldn't mind touching second base. Cutting straight from first to third, though, is right out.