NAD were one of the first British companies to do this back in the late 70s/early 80s (a few owners ago, they are now still Chinese made but with a Canadian holding company in control), and it was for reasons of cost alone (and one of the reasons NAD gained a reputation that still to a certain extent survives of having audiophile products at a cheap price - although they are no longer the cheapest for that niche).
Aside from a large and compliant workforce (the trade unions are state controlled - although recently cracks have been showing following labour unrest and some unofficial strikes), the state subsidised factories allowed for a cheap production cost, and so as well as 'British' hifi being made there you also have most of the Japanese brands produced there too.
And quality on the whole isn't bad (and it has surprised me that the Chinese own label stuff, such as Opera/Consonance isn't more popular), but that doesn't excuse those brands who took advantage of what was largely an exploited workforce from still trading on their 'British' or 'Japanese' status just because they may still have a head office in those places and employ a few local engineers and designers.
Also of course much of the stuff actually gets made in the same factories regardless of what brand label is on the front (speakers are probably the worst for this), so again the company logo can often mislead.
Hifi isn't alone here of course, if you take something like drums whereas you still have companies like the nominally 'British' Premier, 'American' Ludwig or Gretsch, or 'German' Sonor, nearly all their lower to mid-level kits come from the same Chinese factory.
There are still some hifi companies that avoid exporting production of course, with brands such as Sugden being a very good example, but even they have to source most of their electronic componants from Chinese factories before the build even starts...