Quite why this has occurred is a subject of occasional argument, but I've never heard a definitive theory on it, and it partly overlaps with the general decline motivated by ecommerce. They do compete on some level with existing businesses, as debated here, but the more curious impact is they completely alter the character of an area.
Ten years ago they used to be fantastic for obscure finds because it seemed people had not caught on, but these days they tend to be subpar, which is probably a major edge the non charitable enterprises have exploited.