Two words: stopping power. And also the fact that if you aren't hit in a vital area, it's doing lots more pain anyway.
Anyway, I always heard the deagle was a bit overblown. A bit unwieldy, with high recoil, even compared to other magnums. I'm not sure but I think the round itself doesn't travel very fast (relatively speaking).
Bigger isn't always better. Earlier assault rifles like the G3, AK-47 and M14 fired 7.62mm rounds, but in the modern countries that used them they've mostly been replaced by rifles that fire smaller rounds (5.56, 5.8). Aside from obviously having less recoil, smaller rounds can travel faster, and tend to tumble more once they hit soft targets, creating nastier wounds.