Because we've lived through the iterations of the MacBook Pro, and although the Core Duo ones were really hot, the heat issues have remained throughout the updates with newer components.
There is nothing to suggest that even if they kept the same thickness as the current generation, the heat wouldn't be as bad. And if they made it thinner, then we're just going back a step in regard to heat control.
Heat control, most likely structural integrity, and internal specs.
Three things a pro laptop should absolutely under no circumstances bring upon itself. Heat would be forgivable if we were getting an impressive GPU and large internal HDD, but durability and performance are not something a pro laptop should ever even dream of sacrificing...