Firstly, Broadcast Lenses are designed for constant video use of at least 25fps instead of sporadic still picture use from an SLR. So, the design is much more vigorous since any minor mechanical shifting in the broadcast lens would not be perceived when taking stills but would be possibly unpleasant on a Film or TV clip with constant motion. The broadcast lenses don’t ‘breath’ (change image size while focussing) or if it’s a zoom lens they won't change focal length if zoomed in or out. The entire interior structure is tremendously well engineered with high quality metal parts intended for a lifetime of professional usage. The elements are high class, high refraction glass. The lenses are primarily intended for performance, then price, in that order.
Broadcast lenses are also designed to be focussed manually, and the amount of lens spin per shift in focus is firmly managed. If an object is moving towards the lens at a measured pace a broadcast lens will attempt to have the focus barrel also move at a constant rate (within reason). Furthermore, a human hand can only turn so far in one rotation during a single shot so a broadcast lens will also have a smaller throw for the focus -- a limitation not required for a stills lens. A still lens will often have an economically priced autofocus system and a ‘manual’ focus ring which can’t match the speediness and sleekness of a manual follow focus setup specifically for broadcast or cinema use. A broadcast lens with a decent operator can manually focus an unbelievable range at various speeds. It’s difficult to produce a top-quality focus if you are attempting to focus with an automatic focussing setup via a ring of some sort. Still lenses are intended to quickly focus for still photos only. In the case of a moving image, you may want to focus at whatever speed is needed. There is always a messiness which goes with trying to focus moving objects on a non-mechanical setup. The majority of Broadcast lenses are a lot more costly because they are just better constructed and optimized for film or TV work. Many people use still lenses for budget video purposes though, plus they do work, and are cheaper, though not recommended at all if you have funds more professional gear. You simply have to ask yourself “Is this the right tool for the job if budget allows?”