Would be nice to have explained why it is harder? As far as I can see it is just another transform to apply in processing.
The issue for me is essentially that I'm much in favor of having a parametric workflow. Once I started developing an application that essentially was similar to Lightroom, but much more limited in scope. Once Lightroom was released I realized that I would never achieve a similar functionality in one or a few life times. So my project went on the scrapheap and I have been a happy Lightroom user since than.
For me the parametric nature of Lightroom is essential and so is the data base based approach.
Now, as I see it, there are things that are best made in Lightroom and some that are better made in Photoshop. Photoshop extends more in image creation, I don't necessarily see that the two products compete. Lightroom also has some competion, Aperture from Apple but also the new Bibblepro 5 from Bibble labs, which may have some potential.
The parametric local edits we can do in Lightroom are a godsend, and the "automasking" feature is great help achiveing subtle burn in and dodging effects.
A "technical" reason? No, no technical reason at all–other than the fact that doing lens corrections parametrically is a LOT HARDER than simply doing it to pixels.