JackpotCity is regulated by Malta and powered by Microgaming casino games. Both Malta and Microgaming do not accept citizens from the United States of America.
I sent an email to the JackpotCity support to get a clear explanation. Here is their answer :
I regret to say we no longer take US players,
As part of the US gambling law, JackpotCity preferred to step back from the US Markets. However, some online casinos still continue to accept US citizens. Check our top 3 US Casinos.
Technically, you could play to JackpotCity if you signed up from abroad. For instance if you are in vacation in Indonesia or in South Africa, you could play to JackpotCity while you drink your Caipirihna. However, you should not use a US address and an american credit card (from Wells Fargo for instance).
This is the same problem as "signing-up from abroad". You could use a VPN to play to JackpotCity, but you would need to have a real address outside of the US (with rent receipt at your name for instance). If you have a cousin abroad, for instance in the Australia, it could work. Otherwise, in case you win a $3,000,000 jackpot, JackpotCity could block your funds..
We know that Bovada is currently accepting US citizens. It is powered by Top Game software and offers a nice welcome bonus.