Speaking to Search Engine Land, a Google spokesperson said the Google+ posts were removed from Knowledge Graph cards to provide more consistency. Recently, Google has also stripped Google+ branding from its search homepage and notification tray.
Knowledge cards are found to the right of search results on google.com, and display as containers that provide topical bullet-points about a company like stock prices, phone numbers and social profile links.
At a Google Photos event held after the Google I/O 2015 keynote, Vice President of Photos and Streams Bradley Horowitz said the company would be focusing on connecting people via Google+ rather than pushing social as a brand moving forward. It seems this effort is another move in that vein.
➤ Google+ Brand Posts Have Been Stripped From Knowledge Graph Cards [Search Engine Land]