It's not about mechanics, it's about swing path. Bruce's swing path produces lots of high fly balls. When those don't go over the fence, they go for outs too often. Of course, that swing path is one reason Bruce does hit 30 bombs every year. But, the point was, if he wants to improve his average, lowering his hands is going to likely take a slight bit of loop out of his swing, resulting in (likely) less of those types of fly balls.