I think its the other way around, there's a dominant two party system because of the simple majority requirement. If there's a requirement absolute majority, then neither major party wins and the smaller parties and independents that are currently considered spoil sports become very important to form coalitions. That then gives rise to more parties catering to specific interests, demographics and minorities which enable more representation in the senates/parliaments fighting for their primary causes, instead of having to choose either of two always, and any other option basically relegated to a spoil sports.