Because these agencies are horrifically corrupt beyond any usefulness. These agencies could go after any number of human and drug traffickers and make these problems nearly vanish almost overnight because they collect practically all of our communications. But they don't do that. They are used as targeted political cudgels when its handy and when there is much money to be made.

The essence of a canary is you can't say they did, but you can stop saying they didn't.

In the U.S., national security letters (NSL) typically include a nondisclosure requirement:

<https://en.wikipedia.org/wiki/National_security_letter>

A warrant canary asserts that no such obligation has been incurred.

<https://en.wikipedia.org/wiki/Warrant_canary>

Gag orders.