Several reasons. One, a lockdown ia neccesary though possibly not sufficient for containg Covid-19. Two countries seem to have succesfully contained the virus after a large outbreak. These are China and South Korea. Both of these countries combined some form of a lockdown with extremely agressive testing. SK has done more testing while China has been way more agressive with a lockdown. Italy's lockdown is not as strict as China's. Further, when Italy first tried to lock down the north, many fled south carrying the virus with them. Second, we may be seeing benefit from Italy's lockdown now. The daily increase in infections in Italy just fell twice in a row for the first time ever. If you look at their infections on a logarithmic scale you can clearly see that exponential growth is ending. This is a very good sign that the lockdown is working.
south korea seems to ramping up again (slowed a bit past two days), singapore and hong kong both seem to be on exponential trend in spite of similar measures
A function of increased testing in direct response to the long incubation period. Symptoms don’t show up until much later, I think 10 days (you can look it up) at which point people are allowed to get tested. Add a couple of days to get results and it follows China’s trend of ~12 to 14 days after lockdown until peak cases (and deaths?)