Most 7 second mustang I've seen are the old fox bodies which are lighter than any Viper. The Viper itself isn't exactly a lightweight car, this is a pretty wide misconception that I've been seeing lately.
Besides that, once you get the weight down (whether it be a fox body or any other car) the only thing that is left is what kind of power you can put on the ground while maintaining traction. Improving your power to ground is going to come from upgrades, and just like everybody else above says, it's going to be drastically cheaper to do it on a mustang than a viper. So, yes, you're going to see more mustangs (and many other cars) running 7's, 8's, etc than a Viper. It's not that a Viper can't do it, it's just that Vipers are a lot more rare than most productions cars and it's gonna cost tons more (headwork fbody = $250-$500, headwork viper = $3000+).
Furthermore, just because it says "Viper" doesn't mean it should be able to dominate every other car that costs less in every category. If you're going to buy a car for a specific purpose (ie, drag racing) you can't just go for a supercar and expect it to be the best solution (even if you do the same type of mods to it that you would a mustang). If you're modifying any care to run 7's, there's a hell of a lot more to consider other than displacement and initial power (which you referred to in your post). Two dramatically different cars that both have 400hp will run very different 1/4 mile times depending on setup.
You see a lot more mustangs doing those times because:
a) The mustang is not a bad base car to start with if you're looking into 1/4 mile times (cost, chassis, etc)
b) The mustang isn't TOO costly of a car to upgrade when scaled with other cars (I'd rather use an f-body though)
c) There's an absolutely unnecessary amount of mustangs in the world, and it's simple numbers you'd see more of those around than a Viper even in racing (not to mention that manufacturers will create a lot more mods and options for a car that has those kind of production numbers)
But alas, uou can put a rocket on a tricycle and run 7's (or end up dead) but guess what, you'd still go home on a tricycle (or in a hurst). I'd rather be in my Viper.