Because only PlayStation3 supplies DualShock3 compatible electricity to charge the controller! I used to charge my DS3 off the MacMini under my TV, now the PlayStation 4 charges all my DualShock (and SixAxis) controllers.
Monoprice has now released a play & charge kit that is 750mAh for X1. In their text they say 650mAh though, hmmm? I will stick with my Eneloops, I like the idea but not the capacity. Price is easy enough to buy two I guess.