The GOP was pretty much forced to accept him due to their rules(and the other candidates, lol). This is a large reason why there is and has been so much pressure against him in the GOP. The problem for the GOP is it would have taken a major rule change to stop that train which would expose them as not being representative of their voters.
The DNC were better at staving off the outsider. This Was shown by them letting a Clinton run virtually unopposed.
The GOP embraced him as soon as he became the presumptive nominee. They could've tried harder to oust him and they didn't. After that one tape leaked Preibus started getting calls from GOP politicians begging him to cut ties with Trump and pull funding from his campaign. He didn't.
Bernie got closer to winning than anybody expected. Clinton didn't "run virtual unopposed." The reason she won was because nobody else in the party (except Bernie) had the clout or experience to court donors and win over voters, and even Bernie only saw success because he ran an unorthodox grass-roots campaign that spoke strongly to a base that was already reliably Democrat. That's fine, but does dick for you in the general.