The fact is Harry and Meghan are very popular outside of the UK. The Commonwealth love them.
"The Commonwealth"" is a big place. What evidence do you have that folks from Canada to India to Fiji 'love' them?
The rest of the Royal Family are barely known or cared about outside the UK.
Well, firstly, that is complete nonsense. The British royal family are the most famous family on earth. Vastly more famous than, say, the Swedish or Dutch royal family. Secondly, since they are in fact the British royal family, being sustained by British taxpayers, it scarcely matters what anyone outside of the UK thinks about them.
It should be noted that Katherine and William are very dull. They are only popular because of anti-Harry and Meghan. They are not popular for their own sake and in their own right.
I don't think they're 'dull', but actually being 'dull' isn't a bad thing for royals. We're not meant to be interested in them as people, but rather in the role they play.
And do you really think the Harkles are popular - if they are popular - outside of their royal connections? Nobody knew or cared who Meghan was before she got with Harry, and people only care about him because he's the queen's grandson.
They will be a footnote in history at best.