Many people have a sort of hierarchy of sins taken from a list curated by society over the centuries. There are even some sincere Christians who have a sliding scale of sin severity based on wisdom and traditions of men. The Bible cuts through the clutter of human opinion. It explains precisely what the unforgivable sin is and how to deal with it because ignoring it has eternally tragic consequences.
They won't repent
This writer believes there is a direct correlation between the diminishing popularity of the repentance message today and the ultimate widespread rejection of repentance by most during the Great Tribulation.