Terrorists who are competent harden their communications to avoid US sigint, where the superiority of the US agencies is paramount and well-known.

The US is known to have particularly weak human intelligence networks (few spies actually capable of infiltrating foreign terrorist groups). Al Qaeda and the rest know that it's incredibly risky to use electronic communications, so the people that you'd want to monitor already know they're being targeted.

Naturally, the US government's response to this has been to bet more heavily on signals intelligence.