As others have said, a Camry or an Accord meets the needs of most of the US market buyers, and it fits their driving style. It handles well enough to get out of its own way, can carry a family and their stuff fairy well and won't scare anybody when it merges onto a highway. Look at what the Minivan did to the Porsche in the GRM article. Modern cars are high speed, reliable juggernauts compared to offerings from 40 years ago. What was thrilling and felt fast in an old sports car with drum brakes and squeaks and rattles from the factory can be done in air conditioned comfort in a Camcord today. The exceptional of yesteryear is the routine benchmark of today, and the vast majority of the drivers on the road don't want to stray outside of that comfort zone.
Success also breeds more success; high volume selling cars will tend to attract more buyers because they are a known quantity. Take a Camry into any service station and they'll work on it. Take an RX-8 into the same shop with a weird hesitation problem and the mechanics are going to flap their arms, scream "low compression" and tell you to take it to the dealer to have a new engine put in.
Add into that that quite a few people buy cars for image vs. actual use. How many BMW M3s make it to the track until they fall down into the used market and get bought by an enthusiast? How may Corvettes ever see track time? The same thing can apply to pickup trucks and Jeeps; I've seen lifted rigs advertised as "never having been off-road" and while some people are just lying liars, I'm betting there are a fair number of redneck yachts that might drive down a dirt road now and again but have never been rock climbing or mudding.
As noted above, given the way the vast majority of the people drive, their comfort levels and the fact that a beige sedan can meet all of their needs and make them feel safe, I don't think there is a large market for sports cars. In today's market, every new model released simply steals sales from the existing cars in the market. Granted, that is just an opinion, and I don't have any hard data to back it up, but I don't know of many people who cross shop a Camry and a Corvette or a Boxster. No, the person who is cross shopping a Corvette might look at a Viper and a 911 and pick between them. Likewise, if you move downmarket, somebody buying a BRZ might look at a Mustang and a Genesis Coupe and choose between them. The sports car isn't going to steal that Camry's sale.
I recently bought a new car for my wife. She was driving a Jeep Liberty (which I hated and she was ambivalent about) and we got a Mazda6. She liked the way the Mazda handled and braked versus some of the other sedans, but we never even considered a Mustang or other sporty cars because two doors just won't work when you have to help a kid in and out of a child seat 4-12 times a day. Other cars that might have been sportier were way out of our price range. Before the Jeep we had a Mazda3 and that worked great until we had to slide a rear facing child seat behind the driver's side, at which point the ergonomics made it impossible for me to drive without chewing on the steering wheel. We wound up with a sedan because it is the ultimate compromise vehicle. It does enough things well enough to have been the staple of the US highway since the post WWII era and maybe even before that.
Granted, I'm looking at sticky rubber and maybe some performance springs for ours, to maybe spice it up a bit. All that says is that we're not the average family, and we still wound up with a sedan.
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