Reason why people are thinking that Barcelona and Arsenal have chance of winning the europa league it because they have good squad that are very consistent. Arsenal is leading in the EPL and Barcelona is leading in laliga , this makes people believe this two teams have the chance to be at the top of europa.
Arsenal is the undisputed leader in the Premier League and therefore many expect them to win in the EL, but Barcelona is a slightly different story, they flew out of the Champions League and although they are now leading in La Liga, this is not a confident dominance, they were able to get ahead of Real Madrid in the last rounds and it is not clear how many will be able to lead in the championship. So the potential for Arsenal is very big, but I'm not sure about the prospects of Barcelona.
The Barcelona team is now most exposed to La Liga. They are currently at the top of La Liga. Barcelona's unfortunate exit from the Champions League. Their main target now is to win the La Liga title, if they fail to win the La Liga title, it will have some adverse effect on their team.