The kosher lamp is something that comes out special for Shabbos, and therefore after you finish reading on Friday night, you remember not to shut off the kosher lamp, and instead you turn it, in order to darken the room.
With the Shabbulb, it is designed to use the very "same" lamps that you use all week. During the week, you will shut the lamp since there is no point the run the electricity all night. I'm afraid, that on Shabbos, sometimes people will forget and shut off their lamp instead of flipping the Shabbos cover, since that is something that they do the other 6 days of the week. (Even more so, if sometimes you are halfway falling asleep over a book or newspaper, and your mind may not be the sharpest at that moment as you are about to fall asleep, and you will forget it's Shabbos and shut the switch.)