Because they disbelieved the signs of their Lord and disobeyed their Prophet Hud Pbuh.
That was ’Âd. They denied the signs of their Lord, disobeyed His messengers, and followed the command of every stubborn tyrant. They were followed by a curse in this world, as they will be on the Day of Judgment. Surely ’Âd denied their Lord. So away with ’Âd, the people of Hûd.) Hud 11:59-60
(As for ’Ȃd, they acted arrogantly throughout the land with no right, boasting, “Who is superior to us in might?” Did they not see that Allah ˹Himself˺, Who created them, was far superior to them in might? Still they persisted in denying Our signs.) Fussilat 41:15
( They argued, “O Hûd! You have not given us any clear proof, and we will never abandon our gods upon your word, nor will we believe in you) Hud 11: 53
لأنهم كذبوا رسالات ربهم و عصوا الرسول المرسل إليهم و هو هود عليه السلام
( تِلْكَ عَادٌ جَحَدُوا بِآيَاتِ رَبِّهِمْ وَعَصَوْا رُسُلَهُ وَاتَّبَعُوا أَمْرَ كُلِّ جَبَّارٍ عَنِيدٍ
(فَأَمَّا عَادٌ فَاسْتَكْبَرُوا فِي الْأَرْضِ بِغَيْرِ الْحَقِّ وَقَالُوا مَنْ أَشَدُّ مِنَّا قُوَّةً أَوَلَمْ يَرَوْا أَنَّ اللَّـهَ الَّذِي خَلَقَهُمْ هُوَ أَشَدُّ مِنْهُمْ قُوَّةً ۖ وَكَانُوا بِآيَاتِنَا يَجْحَدُونَ )
فصلت 41: 15
وَأُتْبِعُوا فِي هَـٰذِهِ الدُّنْيَا لَعْنَةً وَيَوْمَ الْقِيَامَةِ ۗ أَلَا إِنَّ عَادًا كَفَرُوا رَبَّهُمْ . أَلَا بُعْدًا لِّعَادٍ قَوْمِ هُودٍ ) هود 11: 59 – 60 (قَالُوا يَا هُودُ مَا جِئْتَنَا بِبَيِّنَةٍ وَمَا نَحْنُ بِتَارِكِي آلِهَتِنَا عَن قَوْلِكَ وَمَا نَحْنُ لَكَ بِمُؤْمِنِينَ ) هود 11: 53