Because when Trump Jr. takes a meeting with Russian officials to seek Clinton kompromat and discuss sanctions anyone with half a brain understands that is collusion.
When Trump campaign member talks about Russians having hacked Clinton emails which could use campaigns direction for best public dump opportunity then anyone with half a brain understands that is collusion.
When Manafort is meeting with an FSB agent and passes off to him internal campaign info then anyone with half a brain understands that is collusion.
When Trump is running on anti-Russian-sanctions, is publicly flattering Putin, publicly defending him murdering journalists...all while trying to secure a Trump Tower Moscow project that is fairly straight forward collusion.
When Trump's buddy Roger Stone communicates with Assange on release of hacked Clinton emails, that's obviously collusion.
If collusion of that sort was actually illegal Trump, his son and many campaign members would be in jail.
But it's not and what they were specifically investigated for by FBI and Special Prosecutor was CRIMINAL CONSPIRACY, a much higher level of co-operation which was not proven, possibly because Trump was criminally obstructing investigation by pardon dangling key witness like Manafort and Stone.