Why are the hearts sealed up?
The sealing up of hearts is not the cause, it is a result. A heart is not sealed up because Allah wants it, but because the person is not a believer. Nevertheless no hopelessness is in existence for example “It is sealed up, there is no way back any morer!” Despite the sealing up, the possibility of remorse is available, so long as one has not died.
“But for their violation of their covenant, and their denial of God’s revelations, and their killing of the prophets unjustly, and their saying, “Our minds are closed.” In fact, God has sealed them for their disbelief, so they do not believe, except for a few!” (Quran, an-Nisa 4:155).
After all, Allah had sealed up the hearts of such people, but if only a few wanted it, they could open up from the sealing up. The reason for the sealing up of the hearts other than denial (Kufr), to be mentioned is arrogance of violent people and (Quran, al-Gafir 40:35), attacks from boundaryless aggressors (Quran, Yunus 10:74) and unknown acts of war.