Why do railroads use timber ties on bridges and concrete ones on the ground?

You're assuming that all ties on the ground are concrete, here. In general, all railroads have timber ties, regardless of where they are, bridge or otherwise. However, new high-speed passenger trains are only capable of reaching their top speeds if the ties are made of concrete. Therefore, certain main passenger corridors have been rebuilt with the concrete ties. To switch to concrete ties on a bridge, it may involve completely rebuilding the bridge, or at least a large potion of it, and some railroads aren't willing to invest in that. If it's a relatively short bridge, the train should still be able to cross no problem. Plus, concrete is heavier than timber, so the bridge would have to be reenforced to support the extra weight.