It's a form of "analog" compression. At the time that TV was invented, it simply wasn't possible to transmit all of the scan lines at once, so they cut the number in half. NTSC is a quick succession of alternating frames of even- and odd-numbered lines. If you look up close at an older large screen television, you will "see" the black lines (or the ghost images of such.)
HDTV has a similar problem. The highest spec calls for 1080p (progressive), but at this point in time, we are only capable of broadcasting 1080i (interlaced). Oddly enough, "interlaced" material is actually more of a bandwidth hog with digital transmissions than it is with analog. I won't get into the details unless you really want to know.
