Barcelona didn't fully capitalise on Real Madrid losing to Valencia, but at least they managed to increase the gap by one point, that's still something.
Real Betis is a tough team to play against. Before yesterday, they were on 6 league games win streak (Barcelona had 9 consecutive wins) and that include win over Real Madrid.
Still, with such dominance, i.e. over twice as many shots on goal, shots on target and possession of 74%, Barcelona should've won that, but it is what it is, results are not always fair.