Bookmakers don't care about strength of teams, but about money bet on each side. If you see 5:1 odds for team A against team B, chances are team B has 5x more money bet on it already. Apparently Croatia had less bets as a percentage of all remaining teams after the game with Japan.

Many people in here treat football clubs as corporations that need to earn money. Most, if not all, privately owned football clubs are just advertisement/whitewashing platforms for their owners. They were never meant to earn them money, in fact it's kind of expected to loose money on those in return of good publicity. And that's not even limited to English clubs only. The most recent examples of those are PSG, City and Chelsea, but you can go as far back as Berlusconi's Milan in the 90s or Umberto Agnelli's Juventus in the 50s and 60s, all of the state-founded clubs in the late 40s and 50s by the respective communist parties (any variation of CSKA or Red Star/Steaua in the eastern block - army clubs).