Because WW2 was incredibly expensive, and colonies were more trouble than they were worth.

" colonies imposed heavy and unfair burdens on British taxpayers"

Patrick K. O'Brien

Adam Smith was way ahead of his time

" After all the unjust attempts, therefore, of every country in Europe to engross to itself the whole advantage of the trade of its own colonies, no country has yet been able to engross to itself anything but the expense of supporting in time of peace, and of defending in time of war, the oppressive authority which it assumes over them. The inconveniencies resulting from the possession of its colonies, every country has engrossed to itself completely. The advantages resulting from their trade it has been obliged to share with many other countries." http://www.let.rug.nl/usa/documents/1776-1785/adam-smith-from-the-wealth-of-nations-1776-the-cost-of-empire.php

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A similar question on the cost of colonies was asked here: https://www.reddit.com/r/AskHistorians/comments/7b5dgm/did_colonies_cost_more_than_the_value_they/