Tom,
Because policy in the US requires 505.01 ONLY and 506.01 doesn't meet 505.01.
A rather silly parallel: The US postal service is capable of shipping bottles of acid in cardboard boxes anywhere in the world. They have the system in place, the trucks and airplanes, and trained people. But they won't. It's not about the capability of the system to ship the acid, it's a decision made to not introduce this product into their system.
One of the qustions you've asked is about the difference between the US and Euro equipment. Please take a quick scan of the paper I posted a link to. It's a view of the oil world from the perspective of Shell and the BP/Castrol's R&D lab in Panghbourne, England.
Near the end of the paper is one of the differences between the different markets - notice the fuel economy test descriptions? See the difference between the US and Euro tests? That's just one difference.
Please don't read-into what I've reported from VWoA. No one said that 506 wasn't good stuff - no one said that it wouldn't work in the US. From a technical standpoint, I don't disagree with anything you've said.
The SOLE purpose of the conversation was to discover VWoA's view of engine warranty.
Pretend you're in charge of US warranty at VWoA - it's your paycheck on the line: It's the late '90s and you've just started selling TDIs in the US. (It doesn't matter when they started selling TDIs for this example.) These engines have a long history of running hundreds of thousands of miles. You start replacing engines under warranty between 40,000 and 60,000 miles - rings are stuck, power is down, wear is up. It's costing big money! The US petroleum oil is not good enough! Quick! A TSB! 5W-30 synthetic that meets VW 505.00! Oops - new TSB - it's got to be a 5W-40 synthetic that meets 505.00.
(Watch, shaking, as CNN announces that Benz was hit with a $32 million class-action lawsuit in the US, with another to follow in Canada, because they didn't properly notify owners that the oil life recommendations were based on synthetic oil...)
Fast forward to the small oil pan Passat with the 1.8T...same thing - sludge, engine failures, $$$, TSB - synthetic 502.00, cut OCIs in half, and pay for an extended warranty!
VW has taken some hits in the US market because of oil requirements. They've found that 505.01 works here. They have the warranty under control. It's proven and safe. And they still haven't gotten all of the dealers 'in line' with requirements -- why in the world would they want to add another oil spec?
Andy