racing has a special exemption and some UK firms bet into the US pools... but either way, Betfair don't get involved with the US, the liquidity on US racing works without them as the timezone and information available works well. And US racing is heavily divided between states and various racing associations. HK is a united front and very anti-exchanges at this point.

HK racing into the UK where most of BF's customer base is wouldn't work due to the time and broadcast issues - the international race broadcasts are delayed 5 mins specifically so that BF can't operate in-running on them!