Gloves create a slip surface. When your bare hands grab the bar, the geometry of your fingers and friction from your palms keep the bar from falling. When you wear gloves, friction from your palms work against the gloves instead of the bar, and friction between the gloves and bar help keep the gloves in place. But the gloves themselves are not infinitely stiff. They stretch. So even though the glove-bar contact point may be very stable, the hand-glove contact point will be much less so, because gloves are generally made with a material designed to be comfortable, not to provide maximum friction. Otherwise, they would be made with sandpaper on the inside. So the gloves both slip down your hands slightly, as well as stretch, both of which combine to make your grip on the bar looser. In addition to the slightly wider radius of the bar, all of these add up to disadvantageous geometry/leverage/mechanics.
If the gloves were made of a non-stretching material, like solid aluminum, and the insides were textured to provide as much friction as the bare bar (knurls carved into the inside palms/fingers of the gloves), then they would reduce your grip strength by a smaller amount (but the increase in bar diameter is inevitable). Of course, gloves like that would all but defeat the point of wearing gloves in the first place. Straps > gloves, but increasing your grip strength is probably best of all.