How are state wiretaps obtaining plaintext from encrypted transmissions?
Many reasons, including:


federal law requiring companies (e.g. telephone) to surrender encryption keys
interception before encryption (it's pretty easy to eavesdrop on keyboard input purely via electromagnetic radiation even several meters away)
MITM on an insecure channel (quite effective)
outright broken encryption algorithms
social engineering to recover key
Van Eck phreaking
weak keys
if all else fails, save the ciphertext for until you can subpoena the suspect and retrieve the key (but I guess this doesn't count)


etc... when you are being heavily monitored, purely cryptographic measures become too intangible and no longer suffice. You need to employ physical means to remain in a secure position.