Like anything with cars it's purely a matter of fashion, ego, and perception. Noone who isn't making a purely emotional decision buys a new car. Adding a thousand or two and 5kg is completely irrelevant. If people don't percieve an extra 10-20 miles a day as worth the ugly blue rectangles, then the one with them could be 200kg lighter and $5000 cheaper and it still wouldn't sell.
There's no innovation here. The economics of solar panels are not making them any more efficient, just cheaper. The ideal aerodynamics were solved long ago. This car that Cal entered in the Solar Challenge 2022 is substantially the same vehicle they entered in 1997. https://calsol.berkeley.edu/
Also "optimal sun conditions" obviously means keeping your car in direct sunlight the entire day. That rules out putting your car in a garage, which consequently rules out most urban use where short distances would be sufficient for most use.
So mass adoption of solar cars would discourage tall buildings and tree-lined streets.
I don't know anyone who would want to buy the car in that article. I know plenty of people who have EVs, and I have one too, and still I don't think a single one of my EV-owning friends would buy that car. It's too compromised.
What makes a lot more sense is to put the solar panels on your roof and use them to charge a normal EV, all of which are better at being cars than this thing is. Doing so can also reduce your power bills, heat a swimming pool, and even charge a battery that can power your house during grid outages.
Solar power is great! But the panels don't need to be attached to the car to be useful.
That's why this is not already normal.