I suppose that would make some sense of all this but why wouldn’t it state something like that instead of just “Your account is ineligible to participate in the Twitter Ads program”? I also can’t find anything that states this so I’m a bit baffled. 
That said, it looks like I can now get into ads.twitter.com and when asked for my location and time settings I used USA and Denver Colorado (my primary audience for my twitter account is the contract employees of the US Antarctic Program, many of whom live in the Metro Denver area and my current location, New Zealand, is not my permanent location as I will be travelling extensively.