No specific reason has been given for the change of heart. A GM spokesperson says only that "it's been off the table for quite some time" and that "it wasn't the best approach" for the Malibu. The (undoubtedly significant) costs associated placing comparison vehicles in the 4,000+ Chevy dealers was said to not be a factor in the decision.
GM still plans to promote the hell out of the Malibu. After all, it's set to begin a $100M ad blitz to launch the sedan, and you can expect the automaker to emphatically make the case that it is a better pick than the two class-leading juggernauts, Accord (also brand-spanking new for '08) and Camry.
We wonder if the Chevy program was shelved because a simple Camry vs. Malibu comparison would have been seen as showing less confidence in the product than the 3-way that Saturn dealers ran. The Saturn program's timing allowed it to pit '07 vs. '07 vs. '07, with the imports both being well-known commodities. The '08 Accord's arrival changes things, because after all, it's also brand new, and is likely to make a vastly different impression than the '07, and GM isn't about to act as a supplemental marketing arm for Honda's latest car. Sure, the General might well go ahead with a showroom throwdown between Malibu, Toyota and Honda at some point, but you can bet that no such thing will happen before GM spends some significant time with the newest Accord. Until then, shoppers will have to compare them the old-fashioned way and head to the cars' respective dealerships.