Hamas Misjudged Israel's Seriousness
According to al-Hayat, Hamas believed that Israel wouldn't strike back as it did:
It is also clear that Hamas was surprised by the magnitude of the response, as it did not take the Israeli threats seriously. Despite its decision to resume rocket attacks, it did not take any extraordinary precautions, such as evacuating its security headquarters and equipping hospitals and the civil defense forces. This explains the large number of casualties and the lack of appropriate treatment.