Most times, because most people don't bid for low keyword, its Cost Per Click is always high. Advertisers tend to pay more for these keywords because there are few people that are writing on them.
Apart from these, most low keywords are most reserved for products and companies, giving them leverage to focus on just few users. This has made it easier for the advertisers to pay more for its click.
Apart from these, most low keywords are most reserved for products and companies, giving them leverage to focus on just few users. This has made it easier for the advertisers to pay more for its click.