1- Readiness to commit mass murder & carry out cultural genocide on a truly vast scale- more so than other European powers. No man, no problem- Machiavelli style.
2- The Treaty of Windsor. The Portuguese were used as back-alley thugs by the British on a number of occasions- & the Royal Navy helped them out if things got sour. Actually, the Dutch & French would've wiped out the Portuguese by the 1700s of not for the Anglo-Dutch War & Louis XIV.
3- Massive risk-taking. A lot of private enterprise among the traders- which actually came in more useful for the British & French than anyone else. You'll notice that the Portuguese territories mostly coincide with the buffer zones between the Brits & others.
4- The tolerance the Portuguese had for interracial marriage. A large, flourishing mulatto class is extremely helpful in maintaining a colony- but the French usually had bad luck with this. The British never adopted such a system either. So, there must be something unique about the Spanish/ Portuguese mulatto system...
If there's one central reason- the Portuguese just went along with the British for a rides
