With American roulette, the game comes with an extra number, the double-zero. This increases the house edge of the game to 5.26%. With any other roulette variant using the 37-numbered wheel, the edge will become significantly lower to 2.70%. This makes European roulette more advantageous for players to play.
Here is a visual breakdown of all outside bets in roulette across these two games:
Odds/Even, Black/Red, Low/High: European Odds = 48.65%
Odds/Even, Black/Red, Low/High: US Odds = 47.37%