It's the same reason police (in every country) are always asking for more powers, and then end up not using them effectively. It's a cycle where crime is not perfectly prevented/punished, politicians blame the police, police blame not having enough powers, and then they get more. But the wrong ones to prevent the next tragedy, well, in hindsight of course. So new powers are needed yet again. (And no-one needs to examine why the existing powers are not used effectively, since the underlying problems there would probably be a lot more expensive and boring to fix, e.g. better pay/hours, better management, education, outreach, blahblahblah.)

Then those powers are abused, curtailed a bit, and the cycle starts again.