Why is compression used on bridges?

From a strength of materials viewpoint, most if not all materials are stronger (and less likely to fracture) under compression (where, put simplistically, the forces are pushing the particles of the material together) than under tension (where, put simplistically, the forces are pulling the particles of the material apart).

Bridge designers probably try to put as many structural members into compression, however, as far as I know, any design and especially a truss will result in tension at least at some point, and in bending (never just compression).