You missed Clutch's topic on this...
http://www.imfdb.org/forum/showthrea...t=Desert+Eagle
The short answer is that the Desert Eagle was designed back in the days when Magnum-caliber revolvers were still the most popular weapons on the American handgun market, and it was designed with sports shooters and hunters who wanted a Magnum-caliber semi-auto pistol with a higher capacity higher than that of a revolver. The Automag was basically designed with the same goal, except that unlike the Automag, the Eagle was designed to fire off-the-shelf Magnum rounds rather than the special AMP rounds that had to be made for the Automag. Law enforcement/military purposes were the last thing its designers had in mind.
The Desert Eagle's market was sort of killed by the Wonder Nine revolution of the late-80s/early-90s (when Magnum-caliber revolvers went out of style), but it survived mostly due to its exposure in cinema. That's why Magnum Research so shamelessly promotes the Eagle's appearances in movies (they gave the Eagle its own web site dedicated to its "acting" career).