The UEFA Cup was substituted with Europa League a few years back. In much earlier days Europe had 2 lower tier competitions - UEFA Cup and Cup Winners Cup. The Cup Winners Cup merged with UEFA Cup, before the latter was changed to Europa League.
For any teams that participate in the Champions League, the teams who are eliminated early in the Group Stage may stand a chance to be in the Europa League. The Europa League victor will book a seat in the Champions League the next year.
The Europa League is still being viewed as lower level competition eventhough UEFA made an effort to do a rebranding and assign it points identical to the Champions League. Imagine it as the third place in FIFA World Cup tournament, which people generally do not care about. Furthermore the financial side of it does not appeal to the clubs from the tier 1 leagues in Europe.
As one example of how large the gap in reward money may just be, the winner of the Europa League with a perfect record from the Group Stage gets merely 25% more in reward money than any team that simply took part in the Champions League first Group Stage. If the team could not advance towards second round of Champions League, but qualified for the Europa League as a result of third position in the Champions League Group Stage, there's further prize money to be taken into account.
To paint a clearer picture, in case a team scored 2 points under the perfect score in the Group Stage, but went on to win the Europa League, the prize money that is granted to this team is fewer than any team that participated in the first Group Stage of the Champions League no matter how their results are. From the examples you can see just how the two tournaments are in reality being valued.
In normal cases the two teams that came first and second would be qualified in to the higher level competition next season. The Europa League just isn't as blessed, as being the first runner-up will not secure Champions League seat for the following year.
To cover their monetary interests, many top level teams in Europe choose to get knocked out of the Europa League earlier than to carry on with the challenging timetable of it. Teams from Spain could possibly be the exclusion in this situation because of the domination on the domestic competitions by the 2 largest clubs there.
Competitions such as the Europa League doesn't help the situation in which the clubs from the tier 1 soccer countries are playing too many matches yearly. Lesser clubs in Europe are struggling to keep up with the greater teams and there is no hope of even maintaining the difference.
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