In order to make the Europa League attractive, it shows up in the Champions League, which is the champion of the Europa League, apart from that, there is nothing attractive for big clubs. In addition, big clubs are already participating in the Champions League by becoming champions in their own leagues or by entering the top 3-4. The Europa league and the later conference league (formerly the cup winners' cup) is an attractive destination for medium-sized and small cubes in Europe. If these tournaments had bigger sponsors and bigger prizes, I'm sure the big teams that were eliminated from the Champions League would fight here as well. Football is no longer entertainment, it's a big economy for clubs and countries.
Football style have change right now from entertainment to be big economy or financial side for many team participants, I sure with Europe League seems not excited comparison with Champion League, beside have small team finish under top fourth standing in their domestic league but also small prizes or reward achievement make second European competition is not interested.
Have different participants this season in Europe League after several top team failure lead to Champion League knock out and make them have to play in Europe League like Barcelona, Juventus, Ajax Amsterdam and Bayern Leverkusen. I think UEFA need change regulation about prizes given for the winner in this competition.