Slice the flank steak into thin strips, slice the bell peppers and onion, and drain the diced tomatoes.
In a large bowl, combine the sliced flank steak, bell peppers, onion, and diced tomatoes. Drizzle in the olive oil, then sprinkle the fajita seasoning, garlic powder, onion powder, salt, and pepper over the mixture. Mix well.
Transfer the mixture to the crockpot, spreading it evenly across the bottom.
Cover the crockpot and cook on low for 6-8 hours or on high for 3-4 hours.
Once cooked, shred the beef with two forks and stir to combine with the vegetables.