The reason is that Paris Saint Germain have not lose any match from the beginning till they got to the final, which was the believe people have that Chelsea will loose for Paris Saint Germain to win the title. People need to celebrate the victory Chelsea embraced from the final match because Chelsea they know over three seasons with poor performance, is the same Chelsea that defeated Paris Saint Germain in the FIFA club world cup, those that bet for the favour of Chelsea to win in that final match, won all their bets because Chelsea played the final match to scored three goals to cause heavy celebrations in the stadium, where different countries presidents celebrated Chelsea coach for the wonderful work.
Not only that, PSG has not lost any tournaments this year. They were the winners of the Ligue 1, the Coupe de France, the champions league or any other. While Chelsea was just an average team from England with average performance in Premier League, Champions league, FA cup or EFL. So everyone including myself believed they'd not give a competition to PSG considering how better teams like Bundesliga winner Bayern Munich performed.
The message of the result is football is unpredictable. With so many competitive teams, it gets difficult to determine who would come victorious. The confusion would be huge for Premier League next season, La Liga is surely either Barcelona or Real Madrid but determining who would do better is difficult.