There are two models for the typical locale in which a triathlon works well. Yes, you can put your race in downtown New York City, Boston, Chicago or Los Angeles. But unless you're Dave MacGillivray (Boston) or Jan Caille (Chicago) you probably don't have enough juice to pull that off, and unless you've got a lot of sponsor dollars you'll have a hard time paying the city services bill. No, the better models are as follows...
Where are there scenic places with swimmable water, motel rooms, easily controllable roads, and not much traffic? Seasonal destination resorts, especially those on islands or peninsulas. Examples? Catalina, Hilton Head Island, Lake Placid, Couer d'Alene, Penticton, Big Bear Lake, Panama City, just about every race which is held in Mexico, and most obviously, Kona.
Resort locales need to fill sleeping rooms and restaurant tables in the off-season, and certain triathlons, like the one on Catalina Island, is so financially impactful that this off-season event offers the biggest annual revenue weekend for many or most Catalina businesses. The roads are already sparsely traffic'd in the off- or shoulder-season, and if it's an island (like Catalina) the traffic is easily managed.
If you're ambitious I'd consider this the premier model for a triathlon in which you'd like to see significant growth, because you're able to financially impact a smaller location, yet it's got the infrastructure to serve a couple of thousand athletes.
The other model is exemplified by the Wildflower Triathlon. Just go find yourself a scenic course in the middle of nowhere and make everybody camp out at night. But it's not quite that simple. Wildflower does have electricity. Lake San Antonio has a lot of improved, full hook-up camping spots. Its roads are lightly traveled and the campgrounds almost empty save for Wildflower week.
Personally, I look for the courses and locations in which there are the fewest intersections to control, the least amount of traffic to monitor, I look for course safety, minor displacement of local traffic habits. In other words, I look for courses that are easy and cheap to use, and that will almost certainly represent a welcome addition to the area. It's too hard to place a race anywhere else.