Because there is no trade off, communication is end-to-end encrypted or there is a backdoor.

People are probably considering what you said an appeal to emotion, "Think of the terrorists!" being up there with "Think of the children!" in terms of excuses for misguided policies. Governments already have plenty of tools to track down and deal with terrorists if so they wished: a distressingly amount of planning and radicalization is done over clear text in social media, and even in E2EE there's a wealth of necessarily unencrypted metadata you can make use of.