The problem with saying "NT can just attend other tournaments throughout the year" is that they (NT South in particular) won't. There is a lot of funding that is only available to teams/players competing at national or international level, so while the Nunawading tournament might be a more appropriate standard for NT South, getting them there would be a lot harder.
Also, no one else you're going to invite is a candidate to win the event (except the Victorians). It is VERY rare that SA or NSW get all their teams into the top half of the field, so inviting a weaker team from those states might lift the standard, but it's not going to be by a massive amount.