From the article above:
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or years, Fennell has been lobbying for a law requiring back-seat sensors in new cars, sensors that would sound an alarm if a child's weight remained in the seat after the ignition is turned off. Last year, she almost succeeded. The 2008 Cameron Gulbransen Kids' Transportation Safety Act -- which requires safety improvements in power windows and in rear visibility, and protections against a child accidentally setting a car in motion -- originally had a rear seat-sensor requirement, too. It never made the final bill; sponsors withdrew it, fearing they couldn't get it past a powerful auto manufacturers' lobby.
There are a few aftermarket products that alert a parent if a child remains in a car that has been turned off. These products are not huge sellers. They have likely run up against the same marketing problem that confronted three NASA engineers a few years ago.
In 2000, Chris Edwards, Terry Mack and Edward Modlin began to work on just such a product after one of their colleagues, Kevin Shelton, accidentally left his 9-month-old son to die in the parking lot of NASA Langley Research Center in Hampton, Va. The inventors patented a device with weight sensors and a keychain alarm. Based on aerospace technology, it was easy to use; it was relatively cheap, and it worked.
That was five years ago. The device still isn't on the shelves. The inventors could not find a commercial partner willing to manufacture it. One big problem was liability. If you made it, you could face enormous lawsuits if it malfunctioned and a child died. But another big problem was psychological: Marketing studies suggested it wouldn't sell well.
The problem is this simple: People think this could never happen to them
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And it's true. When you hear these stories, what's your first thought beyond "oh that poor kid". Usually: "how could her parent have done that to her? What kind of parent could possibly forget his/her child in the car?"
Maybe that's not your initial response, but it's very likely somewhere in the chain unless you've read up on it or know someone it's happened to. And that response demonstrates why this wouldn't sell: if you don't think yo could make such a mistake you also don't see a need to protect against it.
Might work if it were accompanied by a public awareness campaign, kind f like was done to get parents to put their kids on their backs. But the actual technology - hell, the products themselves - already exist. People just don't see it as a useful technology without that additional push.