I’m unfamiliar, what’s the trouble in keeping them? I thought they’d long ago evicted the natives, and more or less handed the islands over to the Americans—does this move relieve them of either of those headaches?

Is the idea that Chagossian repatriation now becomes a Mauritian problem? Had the British been taking that problem particularly seriously?

Or more to do with the British not really wanting to be caught between the Americans and increasingly assertive regional powers who may be annoyed by the Americans’ stronghold there?

It's a constant source of legal action and negative news. There's not any strategic need for the UK to keep an island in the Indian Ocean. Might as well get rid of the whole mess for someone else to sort out.