The government's argument goes something like this:
Terrorism is complicated, the police has asked us for this, it will help to keepWhile these may all sound like compelling reasons none stand up to much scrutiny.
us safe, it is popular with the electorate.
The fact that terrorism uses modern technology does not in it self prove the need for long periods of pre-charge detention. It is just as complicated for the Australians, Spanish, Americans etc. all of whom have suffered at the hands of terrorist and none of whom have detention times as long as we currently have.
I am sure the police and security services would love to have much longer to do their work, wouldn't we all. Their requests shouldn't be ignored but ultimately they are not tasked with balancing their powers with civil liberties. Politicians are.
The "it will keep us safe" argument is a very worrying line to follow. Taken to its natural conclusion it could be argued that locking up every Muslim in the UK would keep us safe. Just in case they turned into a terrorist!
Finally, the popularity of an idea at a specific point in time can be a poor benchmark for good laws. History is littered with truly evil acts that were popular at the time.
Terrorist win by terrorising, this manifests itself in knee jerk actions, throwing away ancient liberties and rushing to pass poorly thought through laws. They lose when we have the courage to defend the fundamentals freedoms of our society.