Lifestyle, weather, culture, league position, money, women, not living in Liverpool/England. Same would go for Germany, and Bayern are a huge club, absolutely HUGE. Liverpool don't have a chance in hell of being as competitive in the champs league as Bayern for if everything went perfectly, 4-5 years.
Athletico are almost certain to be in the champs league next season and have a genuine chance to win the Europa this season, they won it last season, are currently second in the league and have an outside chance of challenging Barca for the title(not that likely but Barca have 2 bad games and it gets pretty close). Liverpool, almost certain to not be in the champs league next year, almost no chance of winning the Europa this year, no chance at all of finishing 4th let alone maybe getting second, years away from being a genuine top 4 team, years away from being good in the champs league.