Some people think he is too much of a neocon/interventionist sort for Trump. That's that most common speculation I've heard.
Exactly. Over the last 25 years, Bolton never met a U.S. military campaign that he didn’t want someone else to fight.
Exactly.
Trump campaigned strongly on a defensive, non-interventionist foreign policy platform that stresses reasonable (!) cooperation with countries that hitherto were not on the best terms with us. Trump specifically criticized the disatrous perpetual-war adventurism and ruinious nation building of the Bush/Clinton years.
Bolton is THE posterboy for the failed neocon Bushite adventurism. He talks tough... but he is toxic and unsuited for Trumps agenda, as laid out during the campaign.