Differentiating yourself from the other candidates is necessary, of course.  And sometimes that requires criticism of your opponent rather than just selling your own virtues.  I get that.

But there are greatly varying degrees and tones for how to do that.  It's kinda like how Trevor Siemian would talk about the Oakland Raiders vs how he would talk about Paxton Lynch.  While you want to beat out your teammate for the starting job, the REAL opponent is the other team on Sunday.  In the process of trying to win the starting job, you shouldn't be damaging your own team's chances to win the actual game.  

Same way with trying to win the party nomination.  It means nothing to win the nomination if you don't win the general election.  And that usually requires coming out of the nomination race without major damage, and with the support of the other party candidates and their voters.  A scorched earth strategy during the nomination often carries lingering damage into the general.