Because that data is encrypted and law enforcement is used to get data unencrypted from its citizens. If a judge issues a warrant, they can wiretap your phone, unencrypted data directly from the pipe. If a judge issues a warrant, they can just enter your property, potentially by force, taking whatever they please. They are used to being able to just take what they need for whatever they can get a judge to sign off on.

For years law enforcement agencies around the world have been complaining that encrypted data makes their life a little harder because while they can still take it, they don't know what it is or how valuable it is. They can't use it. So now they push on politicians, they say stuff like "well listen those people who died, we probably had data, messages, that could have prevented that, but they were encrypted." Ignoring the fact that these agencies have far too many messages currently to sort through.

Overall it doesn't make sense to give them encryption backdoors. It's far too dangerous and insane to think that protecting your data requires someone else to have a key. It'd be equal to requiring every physical lock to have a button that says "Only the law enforcement agents can press this" and it unlocks without a key. If it happens, you'll see the UK lose a lot of server business and potentially internet traffic. It's like the UK is intentionally trying to shoot itself in the leg.