The historiography of Hong Kong very much developed at HKU so people who want to research on that specifically do tend to go there. There's a good overview of the origins of Hong Kong history in Chapter 27 of Volume 5 of the Oxford History of the British Empire. (It was written in the 90s so is kind of out of date now as far as the current state of the field goes, but still a good resource if you want to find out about the history of the literature leading up to that point.)

Beyond that there are obvious academic contingencies that mean people who want to specialise in Hong Kong studies generally end up there -- research funding, scholarly environment, etc. Because Hong Kong was a British colony during the Cold War, it wasn't subject to the same kind of pressure that led to the development of the more general field of East Asian studies in the US. Much of that was driven by the US government's foreign and defence interests.

So generally, people who study Hong Kong history as a specific subject tend to be in Hong Kong. Outside of Hong Kong itself, Hong Kong history tends to be contextualised within the broader field of either Chinese history or British imperial history (or both). Historians of China like J. D. Spence have written on Hong Kong, and a popular history of Hong Kong was written by Frank Welsh, who is a British imperial historian.

I know this a kind of unsatisfactory response -- I don't personally know anyone outside of Hong Kong who focuses on Hong Kong colonial history as a thing in itself, and looking up others I wasn't sure about, it turns out they're all at HKU! -- but hopefully that goes some way to answering the question.