And when you do need to carry the charger, it is a lot smaller. (I'm not sure if this is true in all cases, but the charger for my reader was about 1/3rd the size of the charger for my tablet.)

On top of that, readers are smaller and lighter than tablets in many cases. Screen size is one reason (6" seems common for eInk, while the comparable tablet is 7"), but I suspect that the structure of the LCD screen and size of the battery adds to the weight of tablets as well.