it just became too expensive. colonialism transformed as it was realised that you actually don't need to directly rule over people, you can own their mineral wealth and control them politically through debt/finance and geo political means while the governments themselves do the day to day admin of statecraft. European occupation was never liked anywhere, nobody wanted Europeans to rule over them. and it was very easy to unite people against them in rebellions, why deal with that when you can still get all their shit without the cost of rebellions?