At the lower rpm you're not pumping as much oil and that can mean increased wear on the lower end bearings.

 

>>>>>>>>>But, why would lugging it cause one to have to do a bottom end rebuild?

When you consider the amount of force that is exerted on the flywheel and crankpin every time the cylinder fires, you can imagine if that flywheel was standing still and you wacked it with that force as compared to if it was spinning allready , then that force would not be as critical then. This related to lugging and not lugging. I hope I explained it so it is a bit clearer for you.