This is because they opposed God and His messenger. For those who oppose God and His messenger, God is most strict in enforcing retribution.
ذٰلِكَ بِأَنَّهُم شاقُّوا اللَّهَ وَرَسولَهُ وَمَن يُشاقِّ اللَّهَ فَإِنَّ اللَّهَ شَديدُ العِقابِ
zaalika bi annahum shaaqqul laaha wa Rasoolahoo wa many yushaaaqqil laaha fa innal laaha shadeedul-‘iqaab