GM had to agree to kill Pontiac and Sky as part of their bailout agreement with the government. Sales were solid prior to the recession and then fell off a cliff. They were nearing the end of a generation, their manufacturers were axed, and consumers who wanted an affordable sports car either couldn't afford one or were too afraid to buy if they could.

The S2000 was very long in the tooth by 2008 and was killed off for similar reasons. Honda went into full panic mode due to the recession and the 9th gen Civic they designed during that period was the first Civic in decades that didn't make every Car and Driver or Motortrend most recommended list and they refreshed it after a single model year. They went hard conservative and were absolutely not going to redesign an affordable sports car when there was no market.

Fiat was just trying to rejoin the US market after being leaving decades before with an abysmal reliability record. The 124 was a Miata, with an FCA turbo engine at a time where the general sentiment toward turbo reliability still being very skeptical. Why would anyone pick a Miata made by the least reliable manufacturer under an extremely unreliable umbrella that used an engine type many were skeptical of. People were more than happy to deal with the perceived reliability concerns of a blown motor if it was making big power. But a more expensive less reliable Miata? Why?

Del Sol was FWD and a targa. It offered a worse top down experience, was less practical, and less fun to drive than a Miata. The only thing that was better about it was that screaming B series. It had its time and place, and was great. But it's not really even the same type of car.

The MR2 got softer while staying mid engined for the 3rd gen. Its harder to work on, and thus more expensive. I maintain if the MRS got more powerful and performance oriented instead of less it'd have been better received. It was heavier, less powerful, more expensive, and harder to work on than the new NC Miata that was coming out. Who was it for?

All the cars you mentioned had awful luck with needing a new generation during the 08 financial crisis, and/or did the Miata thing, but worse and for more money.

Its not a difficult formula. And other automakers have had success with it. Its not a full 1:1 but the Z3/Z4 did it until it moved significantly upmarket. The Toybarus have been doing it without the convertible option for almost two decades and don't show any signs of stopping.

Honestly the biggest issue that is stopping anyone other than Mazda from doing it right now is the K shaped economy. The number of people who can afford a cheap fun toy car, but not a much more expensive sports car is extremely small. There's more 2 door sports cars >$100k than <$100k currently. Just look at year over year Miata sales and its bleak. They're barely more than a fifth of their peak, with less competition. The working class just doesn't have disposable income anymore. Recent college grads can't afford a newer car. Kids who went into a trade and are now journeyman can't afford anything new, and even if they can they've been brainwashed into needing a truck with big chrome wheels. 40 somethings having a mid life crisis can either afford a 911 or a $900 Lego set, there's no in between. The pool of people who want a $30-40k roadster and can afford it is so small, and Mazda has gotten so good at it it's just going to be so hard for someone else to compete.