Because they only needed a majority to pass the rule change. There wasn't enough opposition to cause a filibuster, which was the point. There was enough opposit

Because they only needed a majority to pass the rule change. There wasn't enough opposition to cause a filibuster, which was the point. There was enough opposition to do a filibuster when 60 votes were needed, now that only a majority is needed a filibuster is pointless. Thus, the minority might as well go home if they face a united front from the majority on the specific points this rule change covers.