Mueller’s group believed that the president committed obstruction of justice. Mueller and his team interpret the law broadly (which is to say, correctly) and they recount numerous actions by the president that interfered with the investigation, and provide plenty of evidence of “corrupt” intent—meaning a desire to protect the presidency from embarrassment and possible criminal liability. Trump tried to cajole or intimidate Manafort, Cohen, and probably Flynn (some information was privileged), among others; and he tried to influence officials like Comey, Sessions, and Mueller himself. Moreover, Mueller (correctly, in my view) does not believe that separation-of-powers and other constitutional considerations give the president license to corruptly interfere with federal investigations.
Whey didn’t Mueller indict Trump? The answer is simple. Under Justice Department regulations, Mueller was bound by the (questionable) OLC decision that a president may not be indicted or criminally prosecuted while in office. While Mueller could have nonetheless announced his belief that Trump violated the law without filing an indictment, he believe that it would be unfair to make such an announcement. Without a speedy trial, the president would be left twisting in the wind, unable to defend himself before a jury of his peers. Thus, “while this report does not conclude that the President committed a crime, it also does not exonerate him.” I would say: the report does not officially conclude that the President committed a crime, but it clearly implies that he did.