Because their fans can be trusted!
There are a number of things to take into account:
1) the police are busy at the moment, football needs police control other sports don't
2) national mourning means silence and the singing of the the "new" national anthem. one club couldn't manage silence on Thursday evening and another club, due at home, have been known not to appreciate the national anthem. I appreciate I am talking about Premier League teams, but easier to have a blanket ban
3) If teams in Europe do not "toe the line" midweek then expect next weekends fixtures to be postponed
4) Her Majesty was the patron on the FA for years and the Prince of Wales is now the patron, so it is right that national game shows its respect by having a weekend off