In addition to its preferential treatment of its own website and the use of its Knowledge Graph, Google also applies algorithmic updates that might further demote the visibility of competing types of search.
Loss of visibility in SERP forces companies to invest in and bid for higher spots on Google AdWords. However, Google’s AdWords policy discriminates against sites which compete directly with other Google services, in particular Google Shopping.
Generally speaking, the inclusion of the term “buy” or equivalent in an ad is considered by shopping platforms and sites to be likely to increase their Click Through Rate (“CTR”) - meaning their efficiency.
Yet, according to the Google AdWord’s policy, Google can reject an ad if it contains a call to action that cannot be performed on an advertiser’s site within 2 clicks of the landing page: 
This strict and arbitrary conditions deprive shopping websites of their best opportunities to compete on a level playing field with Google's services.