It's easier to orgasm in that position. Also, for example I'm a bigger woman, men will fit better in that position if the size of the individual is awkward.
I can have penetration with someone in doggie that I might not be able to have penetration say ... in cowgirl. So for me, it's mechanics.
And it's my guess that most of us LIKE looking a guy in the eyes while we're enjoying each other.
But bottom line is for most, it's just really easy to orgasm in doggie. And a guy can slam harder, pull hair easier and well ... you know, if you're into that sort of thing. Grin.