A few teams, notably Hamilton and Ottawa, should have a footnote--much of their attendance is inflated by School Day Games, and in Ottawa's case, playing games at Canadian Tire Centre, which these teams barely make any money at all as tickets are severely discounted for schools.
In Hamilton's case, remove School Day games and their attendance is closer to the lower tier of the OHL. Normal games it's usually in the 3000s.. which is well below what is needed for the team to break even due to the high cost of operating FirstOntario Centre --another reason why they need their own smaller arena to remain in Hamilton long term.
Ottawa's got a ton of issues that are difficult to list--plus, like Mississauga, have to deal with the NHL in their city.
Other concerns are that many of the games are on TV--plus prices for tickets are too high.
In addition, during the playoffs, attendances are low in certain markets because no discounts are allowed for tickets.