Because SOLT (Society Of London Theatre) is a closed group rewarding its own members. This set of awards is specifically established as a group of theatres getting together to puff themselves. That's their entire purpose. It's nothing to do with celebrating theatre or UK theatre or British theatre. It's purely about the promotion of theatre produced and presented by its own members (and affiliates in the case of one single award!)
SOLT was originally called SWET (Society of West End Theatre) but renamed because of the malodorous acronym, and because "West End Theatre" trumpeted the closed nature of the organisation, whereas "London Theatre" is better at misleading the general public to assume that these are open awards.
They then nicknamed their awards the Oliviers, to associate themselves with someone the general public at the time still recognised as a great actor, principally because they'd seen him in some old movies. As someone posted yesterday, time has moved on, and Laurence Olivier's acting is now more generally perceived as a mannered embarrassment to contemporary theatre. So SOLT will probably soon rebrand themselves again, to perpetuate their dissemblance in misrepresenting themselves as rewarding the best of theatre.
I'd go further than you, to point out that much theatre is international. (I don't mean "international" in the Mark Shenton sense of West End & Broadway). Away from the West End, artists and companies are collaborating across borders, presenting the local globally, and engaging with the world to the world.