In the UK only the top 4 in the Premier League qualify for the Champions League. So at least 2 of the 6 clubs that signed up for the ESL lose out every single year, which means very unpredictable income and matches.
So I perfectly understand why they want a bit more certainty and a guarantee to bring top games to fans every year.
I'm not into football so I understand much less why fans were so opposed to this. And I am also shocked that the PM wanted to intervene within 24 hours of the announcement on an issue that seems completely private and irrelevant compared to all the issues the country is facing.