LA LIGA (Spanish league) specially for top ranked favourite clubs Real Madrid FC and Barcelona FC. Regardless of what you guessed these two clubs played in the championship final until the end. This is a common scenario in La Liga, so it is very rare for a new team to become champions.
This is not a Scenario because La Liga is not regulated by another party. The Spanish Football Federation (RFEF) has set the rules for all teams competing in La Liga, all teams have the same rights, there is no difference.
The reason Real Madrid and Barcelona have always dominated La Liga is because these two Spanish giants are stronger than the other 18 teams. There is no specific prohibition for another team to win the La Liga trophy, anyone can become a Champion and is considered valid by the Spanish Football Federation (RFEF) as long as they are able to finish first at the end of the season.