Cost mainly, though wire beads are sometimes a little easier to install. Because the wire bead is stiffer, it tends to assume and hold its circular shape, and it can be easier to get it properly seated all the way around. The kevlar beads are also sometimes made a little tighter (shorter) so that their slight stretch will not let them go loose. That can make them harder to put on the first time.
Not a huge difference either way. As for weight vs. cost, you might ask whether it's worth 15 or 20 dollars a tire to reduce rotating weight by a net 8-10%, considering the total weight of rim, tube, tire, rimstrip and spoke nipples will be 800 gm or more.