The virus spread _before_ the quarantine became serious enough. People just didn't take the quarantine seriously.
I live in Czech Republic, and I read in the news that some 16.5k Czechs went on vacation to Northern Italy _after_ knowing about the risks. Now we're all in quarantine, with plenty of restrictions. Numbers of infections are still very low, but so is the number of tests that were done.
People just did not take Covid-19 seriously.