One factor to consider is the company and other products surrounding each application. Adobe has a strong background in image editing apps, notably with Photoshop. Apple is a relative newcomer to the field, with other pro-level creative apps, but nothing in the area of image editing.
If one was to speculate (and to be clear, this is speculation) about the future paths of both products, it would make sense to me that Adobe would not want to develop Lightroom to a point where it seriously competes with Photoshop. From a business perspective, they want photographers to buy Lightroom and Photoshop, so it makes sense that they'd look to keep the editing capabilities of Lightroom a good few steps behind Photoshop at all times.
Apple, on the other hand, has nothing to lose. Their goal is to make the editing features in Aperture as good as possible, with no concerns about protecting the revenue of other products. There are user-experience and workflow benefits to be had if they can improve the editing tools in Aperture to a point where a photographer no longer needs to use Photoshop.
Of course, that depends on Apple's ability to deliver. Aperture 3 is an excellent tool, but do they have the skills to be able to keep up with the experienced engineers at Adobe? Is Apple willing to keep up the investment in Aperture at that level?
One could speculate that Adobe will always artificially limit the capabilities of Lightroom, whereas Apple has the ability to steal a march with Aperture.