Because the Obstruction went on while the investigation of the Russian conspiracy went on. (Nice Mueller didn't put them together, but follow the timeline) also they the GOP can't make up their mind what started the report, Was it the Steele report or the Papadopoulos finding out about trash on Clinton? It can't be both. Also Mueller didn't know Barr would be working for the Potus, instead of the Constitution and the people of the US. The GOP are so mean to Mueller and treat him like a traitor. Anything for their cult leader.