The show is so explicit about this. People just blinded cause they like Stringers idea. But the show beats you over the head with examples that it was NEVER going to work like that. It's actually overwhelming to list them all.
Didn't account for Marlo. Tried to reason with him completely misreading him from then jump. Avon scoped him from day 1. "No Marlo. No game."
The show tells the viewer repeatedly that those who try to reform institutions will always lost to the institutions themselves. Stringer, McNulty, Bunny Colvin, anyone who tries to subvert the systems in place lose. "The game is the game
"
Make a long story short. Stringer was a hypocrite. He tried to run the gangster shit like a business and the businnes shit like a gangster. When it didn't go to his liking he violated his own rules, ie. bucking at Poot for rightly calling him out during the meeting or trying to assassinate a state senator. Not hard enough, not smart enough.