The proposed treaty between Britain and Mauritius was conditional on the approval of the USA (due to the UK/US air base), and that approval wasn't given.
https://en.wikipedia.org/wiki/British_Indian_Ocean_Territory
That approval would likely have been considered a mere formality under other administrations, but unfortunately intersected with the current administration's obsession over territory...
I guess all the startup founders must have gone to trump and begged him to stop it.