This is because they believed, then disbelieved. Hence, their minds are blocked; they do not understand.
ذٰلِكَ بِأَنَّهُم ءامَنوا ثُمَّ كَفَروا فَطُبِعَ عَلىٰ قُلوبِهِم فَهُم لا يَفقَهونَ
zaalika bi annahum aamanoo summa kafaroo fatubi’a ‘alaa quloobihim fahum laa yafqahoon