Maybe it could be because the Athletico Madrid players were still tired after they played against Real Madrid in the Champions League so I think it is natural that they had to experience defeat when facing Barcelona even though we know that Athletico Madrid could hold Barcelona to a draw in the match but unfortunately because in the final minutes of the match the concentration of the Athletico Madrid players began to lose control and made their defense weaken so this is what caused the Barcelona players to be able to win the match with a very convincing score.
Don't they have substitutions, Barcelona also played in the Champions League, Real Madrid played and also won against Villareal. Atletico Madrid losing that game was something nobody imagined when the score was 2 goals around 70 minutes, the comeback from Barcelona was done within a space of 28 minutes, if they started well and played 70 minutes of the game with full potential then they should have been able to defend the goals, as worst it should have ended in a draw.
Atletico Madrid is now behind, out of the Champions League and still not leading the La Liga table, Diego Simeone needs to work things out, lot's of time to rest his players and compete very well in the La Liga, next game will determine more about their faith as a contender.
I think it's not just Atletico Madrid who are feeling tired, Barcelona should feel the same way because they are also playing in the Champions League. So it's not because of that Atletico Madrid lost, but because Barcelona did have a better performance than them, and most crucially Barcelona has the spirit to win and is willing to do their utmost to get it.
Atletico Madrid actually has an advantage because they were eliminated first in the Champions League, so they can focus more on domestic competitions, but unfortunately, they could not take advantage of that opportunity.