Why did the victors of WWI not just incorporate the 'conquered' territories to their already established colonial structures, like the British Empire and French West Africa? More specific: In the case of French and British Togoland (former German Togoland), why didn't Britain incorporate the Western part of Togoland into the Gold Coast right after WWI and why didn't the France do the same thing with its conquered territories in eastern Togoland? Why did they agree to 'only' administer these territories in a mandate system if they could have 'had it all'?
I think the broad historiographical consensus boils down to: the creation of the League of Nations itself was part of a shift towards new forms of international law in the period in question. While Britain and France certainly had the power to do otherwise, political leadership in these countries was broadly committed to the development of international law in response to the crisis of WWI itself, and this commitment constrained their own actions in relation to the allocation of former German colonies.
If you want to read more, I'd recommend Susan Pedersen's The Guardians: The League of Nations and the Crisis of Empire (Oxford, 2015).