Because Mapquest is totally unreliable. It has people turn the wrong way on one-way streets and has you drive really out of the way sometimes.The programs are only as good as the people who wrote them and the information used. Google has better information. Better yet, AAA and Bing Maps are, in my travels, better maps than Google.
Google goes straight east to Oklahoma City , then straight south. Mapquest goes diagonally south east...shorter route...1 1/4 hrs difference. I actually find both Mapquest and Goggle often full of errors and close to worthless. See site below. I find it to be far more accurate.