Why? Because Twitter is not Facebook. Why do they need to have the same features?
Yes they are different sites and yes they serve different purposes. However if functionality can work on both and can aid the users of that site then why should it not be introuced - just because you can already do it on one is not a good enough reason. If it was then you would not see competitors coming out daily with new sites that do what existing sites do but try and better it.
Twitter has millions of users who do not have a Facebook account and vice-versa. Providing they don't move too much from what makes them unique then giving their user base useful features can only be a good thing.