das said:
Willpower is WAY overrated.
Not when you're a representative government, no. One merely has to look at the reaction of half the population in America regarding Iraq over a (lets be honest here) fairly low 3,500 dead and the results it has had (and will have) on elections to see this is the case. An even more profound example is President Clinton's withdrawal from Somalia after only 18 casualties in 1993. Public perception has been incredibly strong throughout the later half of the 20th Century, and even stretching back into the first half.
This is evident in the French experience in Vietnam (after the disaster at Dien Bien Phu, being so badly beaten as to have to beg the Americans to deploy four nuclear weapons against North Vietnamese positions around the base) and Algeria (where the Foreign Legion and French army committed numerous attrocities, suffered terrible loses, and at one point even attempted to assassinate de Gaulle himself) and the government was no longer able to truly support the war in the eyes of the people. This is similarly true for the American experience in Vietnam, and the Soviet experience in Afghanistan. All of these particular conflicts also incurred severe economic and military drains as well but those only contributed to the unwillingness of the electorate (or people, in the case of the USSR) to support them and therefore the inability of the colonial (or otherwise) government in question to sustain them in the long-term.
As MacArthur said, "It is fatal to enter any war without the will to win it."
Or, perhaps, "One cannot wage war under present conditions without the support of public opinion, which is tremendously molded by the press and other forms of propaganda."
These apply to not just war but most things involving concentrated effort and use of resources.
To embark upon colonialism is to deal with little guerilla wars and "incidents" every so often. To not be able to deal with those is to ultimately fail. Winning requires resolve, and resolve tends to require being able to overlook things like human rights or rules of war or morality in order to do what is necessary to win the conflict. Europeans, and to a large extent a good portion of the rest of the industrialized world, had their fill of slaughter with two world wars killing a hundred million people or two, and decided that toothless morality was better (and many still continue to do so today in the face of enemies not so encumbered). They couldn't maintain these positions economically or militarily anymore either, and that certainly didn't help, but even if they had been able to, they couldn't have sold it to the people who had the power to install or remove them from power. And so they let it go.
Why? Because Europe wanted (and still wants to this day) to make babies instead of bombs, to work 9 to 5 jobs, to get three years paid leave when having a baby, and so on. They got tired of blood and bullets and went soft.
Take away those two world wars killing those millions and millions of people, and you take away that aversion to bloodlust and that decline into weak decadence, or at the very least, greatly ****** it. And that leaves the colonial option infinitely more supportable, which provides alternatives to the looming American and Russian hegemons, which is what we're setting out to do.