Sevilla used to be a strong club in La Liga, but I don't know what went wrong for them. They even find it hard to dominate their favorite competition, which is the European League competition. On paper, Sevilla is the favorite to win against Celta Vigo, but their inconsistency this season made them not win against Celta Vigo, even though Celta Vigo is also struggling this season in the league.
The thing about La Liga is that it's basically a two-team competition, Madrid and Barcelona. The other teams like Atlético de Madrid, Sevilla, Valencia and some others, depending on the year, do better or worse. It's mainly a question of budget. Madrid (€1.04 billion) and Barcelona (€0.86 billion) have twice the budget of Real Sociedad (€0.48 billion) and Atlético de Madrid (€0.45 billion), who are third and fourth. Fifth is Athletic Club de Bilbao with €0.24 billion, and from there, downwards. If we take into account that in football, as in the economy in general, the bigger team has an easier time raising revenue and loans, we can see why these two teams dominate the competition year after year.