That's because Stasi was still on a "human level". Prims and Tempora (which is just the British NSA counterpart and has been running for just 18 months, see [I just see this message from GLP telling me I cannot quote content from The Guardian here ) are of course done by humans too but have at their disposal advanced information technology...
It is only to be expected that these systems will be yet more perfected as time goes by.