...plus, there was an important synergy:

The Corleones were pre-eminent in illegal gambling. Nevada represented the only legalized gambling in America in the mid-Fifties. By becoming powerful in legalized gambling, the Corleones could leverage and extend their power in illegal gambling. The wire rooms, loan sharking, odds-fixing and other activities that they used to fleece gamblers in NY would help them fleece high rollers in Nevada.

Though GFII was not envisioned in the novel, we learn from the Senate hearings that Michael "controls all gambling in America." And he was angling with Roth to control it in Havana, the other legalized gambling mecca in the Western Hemisphere. That's a result of that synergy.