Traditionally, DDoS is an avenue of profit for botherders. But today's botnets have evolved to include several attack vectors other than DDoS that are more profitable. And just as any business-oriented person would do, attackers follow the money.
Users today want more. The ubiquity and convenience of online competition means that customers want everything better, faster, and cheaper. One key component of the user experience is service availability. Customers expect applications and online services to be constantly available and responsive.