It's not an exaggeration to say that America is hooked on Shark Tank.

The reality-show attracts an average of 7.5 million viewers per episode, according to Nielsen data, right up there with the most recent season of singing competition American Idol at 7.7 million. 

For proof that Shark Tank's popularity extends all the way to small town America, look no further than Wednesday's open casting call in Portland, Maine, a town of just 60,000 people. More than 300 entrepreneurs showed up to pitch show's producers, who have hosted similar casting events in smaller cities like Greenville, North Carolina and Knoxville, Tennessee. Organized by cell phone company U.S. Cellular, the small-town casting calls are aimed at giving more Americans a chance to pitch their business ideas. 

"There were tornadoes in Oklahoma City the day we were there, and we still had a big turnout," says Shark Tank's supervising casting producer Scott Salyers.  

So how did Shark Tank become as popular as American Idol? Here are four ways the show has turned America into Shark Tank nation.

1. The Mark Cuban effect. It wasn't until season three, when Mark Cuban joined the show, that Shark Tank surged in popularity and viewership. While Cuban's celebrity certainly helped, he also brought a new energy to the show that made it exciting to watch. For example, Cuban frequently gives entrepreneurs a 20-second "shot clock" where they have to take his offer or lose the deal--even before the other sharks can weigh in. "He treated it like a real business transaction and it sparked a little fire," Salyers says, adding that co-host Lori Greiner's take-it-or-leave-it approach to negotiating has also made the show more exciting.

2. Shark Tank is a real reality-show. Like American Idol, Shark Tank is a completely unscripted show where the hosts have no idea who they're going to meet next. What makes Shark Tank even more compelling, however, is how the fate of many small businesses depends on getting an investment from the sharks, who are investing their own money. "These entrepreneurs really need the money," Salyers says. "That's where the drama and the excitement comes from."

3. Innovation is infectious. Watching entrepreneurs spar with the sharks on national television is certainly entertaining, but one of the most addictive aspects of the show is seeing the innovative products contestants come up with. Amazingly, some of the most successful products have been remarkably simple consumer goods. Screen door repair product ScreenMend, for example, went from $4,000 in lifetime sales to more than $1 million in a single year. All-purpose cleaning tool ScrubDaddy hit $50 million in total sales since inception, making it the most successful product in Shark Tank history. 

4. Shark Tank's target audience is everybody. Reality-shows like Survivor and Keeping Up with the Kardashians are hugely popular, but they're not for everybody. Shark Tank, on the other hand, appeals to all age groups. Why? Because entrepreneurship is a core American value, and anybody can be an entrepreneur. "A parent is not going to say to their 10-year-old, 'Let's watch The Bachelor. You might learn something,'" Salyers says.

Indeed, Shark Tank's educational value is one of the crucual ingredients that distinguishes it from the long list of celebrity-centered reality-shows. "It's not a guilty pleasure," Salyers says. "It's a pleasure."