I can't speak for anyone else, but a few of my own personal reasons are:
*Really thin cord, combined with cold weather and throw in a little bit of age. Getting harder to tie in cold weather for me.
*Humongous tarp. Speed is appreciated when wrestling a tarp in strong winds.
*Once the initial tarp sag hits, it sure is easy to readjust!
*Nice and easy to center my tarp over the hammock
*I'm short and it's a pain to try to tie knots with my arms way up in the air!
I still use my tautline hitches for all of my guylines though. I actually enjoy making knots