Capsule shadows do not always necessarily do micro-self-shadowing like SSAO can do, so it is used often in tandem with some SSAO. Also it scales on a per-object basis for cost, while SSAO is flat usually based on resolution.Is there a reason why more games don't use capsule shadows? It looks so convincing without all the ugly artifacts of SSAO.
There are other reasons as well - SSAO passes can be used for things other than just the SSAO effect.