The thing is that the top teams in most leagues are rather stable and can plan on playing international football (and reaping the corresponding revenue and prize money) every season, so that they can afford to build large squads geared toward playing in 3/4 competitions. On the #6 or #7 spots, there is typically far higher volatility from year to year, so these clubs neither have the large squad, nor can they build one based on a one-off participation in international competition.