Explain to them that the slow projectile speed and overall lack of range turns the matches into completely different games than either paintball or airsoft. For instance, point out that the range, accuracy, and speed of both paintball and airsoft, plus the required eyewear, make it almost impossible to play a zombie scenario with those games.
Speaking of which, tell them if they want to "man up" they would be playing paintball anyway.
This is the essence of what I was going to say. I am in the military and I get coworkers to play Nerf over airsoft or paintball based on this argument alone. Sometimes we just want to throw realism out the window and actually dodge projectiles, instead of tactically hiding. Also the fact that you can be within talking range of someone and know that they can't hit you with what they are carrying. That's not common to any other wargames (except airsoft, but that's not an argument for this thread). Also, let us not forget the big one; no field fees! You can play Nerf in a park in the middle of the day. It's legal and safe, and if you bring enough blasters, you may even get new people in.
Which brings me to my next point. Some people find the hardest part of "recruiting" just getting people to buy and mod their own, which is why I bring at least 8 people's worth of armaments to each war I attend, and more if I host. Sometimes people need a trial run before they are willing to put in time and money. I don't even know how much money or man-hours have been lost to Nerf. We have to understand this when we are looking for new nerfers as well; some people aren't ready for another hobby.
In the end, though, if you explain how much fun it is, you provide "rentals" for free and they still won't Nerf because it's not cool enough, you probably don't want to play with them anyway. That mindset never goes away. Let them get back to whatever MTV is telling them to spend their time on, because that's what makes them happy.