I'm going to break things down a few levels. Disclaimer: I'm a nerd not a mathematician, so if anyone else can fix my errors that would be great.

Cryptography is a cat and mouse game. There is currently no "perfect solution" so that A and B can communicate and C has no way of cracking the communication at some point.

Cryptography is very complex for obvious reasons, but a lot of modern algorithms hinge on the time it takes to calculate prime numbers and test them against encrypted communication. Traditional PCs take an incredibly long time to calculate prime numbers.

Quantom PCs don't. The way they operate makes them incredibly helpful for calculating primes, that's why a lot of cryptographic algorithms will be in jeopardy once it is more widely implemented.

But back to your question. There are already rumors that NSA is using super fast traditional computers to calculate prime numbers and collect them in a database to make cracking traditional encryption easier.

The only thing I can think about with is is that for the NSA they are not moving quickly enough to catch up or they suspect any future quantum key encryption will thwart any attempts they made.

This would be in tandem with moves by the UK parliament to get a law going that implements backdoors in devices or apps (I assume that must be pushed by GCHQ?).

Personal opinion: encryption with a backdoor is ridiculous. The government likes to represent that they're the only one to access those, but it only takes one savant 10yo interested in penetration testing or one rogue government employee for this backdoor to be used for malicious purposes. And it's not like these ppl already exist.