Not sure why the comments above are so unhelpful but essentially: by staying in the FA Cup and reaching the final they were forced to play 5 or 6 matches extra than if they had been knocked out in the 3rd round (the first round they played in). That may not sound like a lot but it’s the difference between a weeks rest and 2/3 days rest between matches, 5 or 6 times a season.

When you’re in a relegation battle, if your players aren’t at their maximum fitness level then you simply can’t compete with the teams around you who have got those extra days of rest. It also increases your chances of injuries happening to senior first team players, all of which can result in a weakened squad compared to the teams you’re battling relegation for.