The key element is the interaction with EURO 2020. The Nations League runs from September to November 2018, while EURO 2020 qualifying is over 10 match days from March to November 2019. The Nations League Semi-Final/Final is able to run alongside this during June 2019. The EURO 2020 qualifying has become a bit simpler with 10 groups and 2 teams from each group qualifying. The 4 remaining places come from play-offs from the previous Nations League and are held in March 2020. The only fault with this is that the qualified teams only have 3 months to prepare for the Finals.
The Nations League is supposed to be held every 2 years, but how would this interact with the World Cup qualifying? This qualifying normally consists of 9 groups (and 9 groups of 6 teams would be perfect) with play-offs for the runners-up. The qualifying tournament could only run over 10 match days from March to November. This couldn't accommodate play-offs or the semi-finalists from the Nations League, as they would be playing in June 2021.
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