The idea is that IaaS vendors, like AWS, Microsoft Azure and Google Cloud Platform, are expanding to become what some call IaaS+ vendors by adding PaaS-like features to their services. As AWS, Microsoft and Google aim to make their clouds a destination for application developers, the it makes pure-play PaaS vendors less compelling. There could still be a big market for private PaaS though, which would be on-premse deploymnets of PaaS tools (companies like Pivotal, Apprenda and Red Hat's OpenShift play in that market).
The broader point is that cloud users should think about what they're looking to get out of their cloud deployment and which vendor best serves those needs more than trying to choose an IaaS vs. PaaS.
Another 2014 report from DZone shows 58% PaaS and 52% IaaS adoption rates among the companies it surveyed.
SaaS has seen a bigger growth curve because quite frankly, it is easier to implement one or two web-based apps that build an entire ecosystem in the cloud.
Pivotal in a blog post last year (http://blog.pivotal.io/pivotal/p-o-v/10-key-paas-statistics-you-need-to-know) mentions several items to also support this trend:
1. Lines of Business See the Cloud as More Strategic than IT
2. Business Lines are Purchasing Cloud-related Technology Outside of Centralized IT Budgets
3. Business Agility is the #1 Driver of Adoption; Hybrid PaaS to Grow Fastest
Pivotal also cites another reason for the slower adoption rates on PaaS: lack of early openness for developers. "Google App Engine required you to develop in Python only. Heroku required Ruby. In order to get the benefits, developers were locked into custom APIs and restricted technologies." Since those early days, developers have had more choices, but still many PaaS players aren't as open a platform as they could be.