They didn’t detect it until it already spread wide.
As others wrote, they have a large older population, and the virus disproportionately affects older individuals and those with health issues, which tends to go hand in hand.
Because it spread all over, there was a limitation of resources. A limitation of resources means that not everyone gets treated. And, as you can deduce, that means a lot of people will die even in cases where treatment might have saved them.
Because it spread so quickly undetected, there could likely be a large portion of the population that is infected, but not being tested, because they are fine. This would “inflate” the mortality rate a bit. Remember, you can carry the disease without showing any symptoms at all, and you can transmit it to someone else as well (iirc, i would need to hunt for a source).
That is most of the logic for why the situation is so severe in Italy.