Why do people like Black Sabbath?

Because they are an outstanding band, with amazing riffs, melodies, basslines and overall tracks. They are the first heavy metal band, and were the heaviest in the world at the time - and they're still pretty heavy. They have an immensely talented guitarist and have had the best vocalists, bassists and drummers in the business. At the time, they were the darkest band around, and again even by today's standards they're fairly dark, although not in a deliberately antagonistic way. They embody the feelings of youth who feel their futures going down the drain with their doomy, sludgy sound and downbeat lyrics.