TCP congestion control is designed such that, under steady-state conditions, each TCP stream traversing a physical link gets an equal share of the available bandwidth. When an application that uses multiple TCP connections competes with another that uses a single one over the same link, the resulting bandwidth distribution is unfair to the latter. (Edit: there's also another problem, related to bufferbloat, but that one is a lot trickier to explain).
Of course, this may not be a problem when used on an internal network (which I assume is the scenario you are referring to). On the public Internet, however, HTTP is becoming increasingly disruptive to other kinds of traffic due to the increasing number of parallel connections used by web browsers.