While it may not fit the "definition" (not sure who is really authorized to define what the cloud is and isn't), it has gained acceptance by a vast majority of

While it may not fit the "definition" (not sure who is really authorized to define what the cloud is and isn't), it has gained acceptance by a vast majority of people that SaaS is a cloud service. The most growth "in the cloud" is going to come from there. The market for IaaS is relatively limited in size and growth (though it still has tons of room), and PaaS is even more limited. The markets are pretty clear in size...SaaS touches everyone. IaaS touches IT departments and PaaS touches developers. It's not hard to see what SaaS gets so much attention, followed by IaaS and very little attention to PaaS.