Mak’s got it.
If you leave the US market (which is officially what they’ve done) and then a US player hit’s the jackpot, the jackpot would reset, MGS would have say “someone won”, but they couldn’t say where they came from. People would grumble that “something fishy is going on”, “it’s a scam”, etc. PR they don’t want.
That’s the only logical reason i could think of at this point.