That would be because President Obama is an intelligent man and knows where to find good advice and was able to judge what to do. President Obama did not start his term believing that all the advisers were stupid, that all the military were incompetent, which is what Trump says he believes.
Trump, according tho those who know him is an ignoramus who does not read books, who gets his news from Fox and CNN's headline reports and idiotic commentators, and cannot be trusted to tell the knowledgeable advisors from the dolts and the lobbyists who just want to sell more weapons or supply him with more mercenaries, like Blackthug or whatever. Trump has the attention span of a gnat and lashes out at anyone and everyone who questions him immediately.
Fifty military and foreign service advisors have recently submitted a report affirming that Trump is the most dangerous person ever to run for president. No group ever said anything like that about Obama, or McCain or even Juniorbush.
All Trump knows is how to promote Trump. He is clearly unqualified.
And he will lose. Every time he opens his mouth he makes a bigger fool of himself.