Maybe the use of wires instead of aramids is because of the clamps on brakes and derailleurs. Wires are strong both in the direction of the strands but also perpendicular to the strands, in other words it is difficult to cut wire. Aramids on the other hand tend to be very strong in the direction of the strands but not very strong perpendicular to the strands, in other words, easy to cut. This was shown in a GP a couple of years ago when Villeneuve's tire came off in a crash. When the suspension failed, the cable of kevlar that was supposed to hold the wheel was cut off, and the wheel went away and killed a man. The cable was engineered to stand a huge amount of stress in one direction, but perpendicular to the strands it was so weak that the failing suspension cut it of like a scissor. So, to sum this up, the clamps are probably one reason for wire instead of kevlar. The higher cost for aramids and the relative low weight saving is another, probably the real cause.
(Sorry for my bad english but as you might know, it is not my native language.)