WASHINGTON—Robert Mueller’s long-awaited report outlined efforts by President Trump to curtail or shut down his investigation, but it also made clear why the special counsel didn’t pursue charges of obstruction of justice and why contacts with Russians by the Trump campaign didn’t amount to a criminal conspiracy.

The 448-page report, released Thursday with portions redacted, follows a nearly two-year investigation into Russia’s interference in the 2016 election and ties to Mr. Trump’s campaign. Mr. Mueller, explaining in the...