We've asked Google for its take on what happened, and we'll let you know if it has a comment. As it stands, the issue might be resolved by the time you read this, as the cards disappeared entirely during the writing of this article. However, it's doubtful that there's a sinister conspiracy here. Regular search results still mention the missing candidates, and Google has historically shown results for other hopefuls. Most likely, this is an error -- albeit one that's bound to raise eyebrows.
Update: Sure enough, a Google spokesperson tells us that this was the result of a "technical bug." Its search code was only highlighting candidates embroiled in an active primary election. Since Trump and Johnson had already sewn up their candidacies, they were mistakenly excluded from the results. It's still odd that Sanders is included (just because the DNC is ongoing doesn't mean his campaign is active), but things are effectively back to normal. You can read the full statement below.
"We found a technical bug in Search where only the presidential candidates participating in an active primary election were appearing in a Knowledge Graph result. Because the Republican and Libertarian primaries have ended, those candidates did not appear. This bug was resolved early this morning."