In one word...pricing.  Or more specifically increased pricing complexity.  SaaS was supposed to be about pricing simplicity and transparency whereas the software was non-negotiable, pricing was straight-forward (per user), and the vendor maintained a take-it-or-leave-it approach.  Wall Street, in conjunction, embraced and rewarded the SaaS vendors because it has enabled them to build repeatability and predictability into their financial models. 

Today, that SaaS pricing model is under threat of regressive complexity (I'm coining a new term).  Here are two recent examples of that pricing complexity (the respective vendor names have been concealed to protect the innocent).

These are actual pricing models submitted to customers (although I have rounded the prices to the nearest value).  Where is the simple per employee per month (PEPM) structure?  As you will note, the pricing models themselves are beginning to look eerily like their ERP counterparts --- the exact model they are looking to destroy.  It is important to note, in both instances, the salesperson was backed into the corner, one at quarter end, to provide massive discounting for the software.