English clubs regularly field changed teams in the domestic cups, which is treated as a slight against the competitions. The behaviour follows directly from how the money and the calendar are arranged.
The reward structure is lopsided
Prize money for winning a domestic cup is modest next to the value of a single league position, and league positions determine broadcasting payments directly.
Avoiding relegation is worth more still, since it protects several seasons of income rather than one payment.
A coach allocating a fixed number of high-quality minutes across a season is therefore choosing between a large certain reward and a small uncertain one.
The fixture list leaves no slack
Cup rounds are scheduled in midweek between league matches, so a full-strength cup selection costs a team recovery days before the following weekend.
Clubs in European competition face a further midweek commitment, and the compression through the winter months becomes severe.
Rotation in that context is not a choice about which competition matters. It is the only way to complete the fixture list with available players.
Squad rules push in the same direction
Domestic cups permit larger squads and more substitutions than the league, which makes them a natural place to give minutes to fringe and academy players.
Those minutes have real value, since a squad player who has played recently is more useful when an injury arrives in a league match.
The cup therefore functions as the only competitive training environment available during a season that offers almost no training time.
Why smaller clubs behave differently
For a club outside the top group, a cup run is one of the few routes to a large payday and a genuinely memorable season.
Their league position is also less financially sensitive, since mid-table placings carry similar payments and the relegation risk may already be low or already unavoidable.
The result is a competition in which the incentives are asymmetric, and the asymmetry is what produces the upsets it is celebrated for.
What replays and scheduling changes did
Removing replays from later rounds reduced the fixture burden and simultaneously removed a lower-division club's chance of a lucrative second match.
Scheduling cup rounds on dedicated weekends raises the profile of the competition and forces stronger selections by removing the midweek conflict.
Each adjustment trades one problem for another, which is why the balance between the cups and the league keeps being renegotiated.