This book, written by a contemporary scholar, provides insights into the different causes of forgetfulness. He discusses both the spiritual and practical dimensions, how a person's memory can be weakened, and, more importantly, how one can acquire a sharp and powerful memory.
Another reason explained by Maulana Qasim Nanutawi is that the disbelievers denied their Creator. In doing so, they not only rejected Allah but also His eternal attributes. Since these attributes are everlasting, rejecting them warrants a punishment that is likewise eternal.