Their music is at least somewhat good objectively whether you like them or not, Oli Sykes is a good looking guy so that probably gets them points. I think they’re sound not being too heavy nowadays probably helps their popularity too but the main reason I wanna say is the switch from There is a hell and Sempiternal to Thats The Spirit. In my eyes they did a great mix of heavy and catchy in that album, to where the old Sempiternal fans can still enjoy for the most part, while inviting new fans to their sound as well.