I mean...it's understandable to a certain degree.
I went into this election ambivalent about Bernie. I liked him well enough. I certainly didn't dislike him. I wasn't ever going to vote for him, but, sure why not? But, the way in which he ran his campaign was utterly incompetent and off putting. As someone who has been active in the Democratic party since I was a kid, I found a lot of his campaign's behavior to be indefensible.
Hillary has worked very, very hard to earn the votes of people. Bernie has been defacto eliminated since March 15th, but he continues to lob grenades at the party, and at Hillary because he can. When we live in a country in which the alternative to Hillary is a literal fascist, I think it's understandable that people's patience for Bernie's theatrics has come to an end.