In another comment you fail to understand the basics of traffic analysis. Bitcoin transactions can be performed independent of routing and latency constraints. You can encapsulate them in infinite layers of onion routing and they will still function.
The same can't be said of voice traffic.
Silk road is vulnerable because it needs to physically deliver product. The SWAT team can order product and then arrest the delivery boy. It can coerce the delivery boy to tell them where he got his package, and so on.
Tor is not the weak link here. Tor is amazing, but the price you pay is latency, which is death for some kinds of applications. Bitcoin is not one of them.