Well, first of all it's an elimination game. There's games where everybody plays until the game ends, and then there's elimination games like Monopoly or Risiko! where people have to stop playing while their friends are still at the gaming table (well, supposing you still call friends the people you want to play Monopoly).



Second, it is long, and common house rules make it longer. This includes not auctioning (if you hits an unsold property and don't want it, auction it. Instead, people pass, and it takes longer to deal the properties and to be able to strat building).



Third, if any player stops trading, it becomes less fun for everyone.