....It's a fuel economy issue. To meet CAFE, all the manufacturers are reducing weight. Spares are heavy.
Also, as tires got bigger and bigger, there's no place to put that flat 20 inch wheel and tire in your car anyway.
One of the reasons I decided to buy my 2004 Boxster S "50 Years of 550 Spyder Edition" was that it was the last Boxster with a spare tire. And the last Boxster with an oil dipstick.
I had the sidewall blow out on my driver's rear tire at Boxstoberfest last year and I limped 50 miles to a tire shop and just got there before closing time on a Saturday. If I had not had a spare, I wouldn't have been able to drive home until Monday because all the tire shops were closed and I was out of cell service south of Junction, Texas.