For the same reason modern econoboxes don't get the super high gas mileage of the same cars back in the70's and 80's: modern safety standards have just made everything too damn heavy. Air bags, crash test, all that crap adds weight. The Solice platform is a perfect example. A small car that is still a heavy pig.
Cars have also gotten so expensive with fuel injection computers, air bags, lifetime emissions warranties, that trying to save money by eliminating all the bells and whistles just makes the consumer feel cheated. The base cost of the car is already so high that the buyer is left thinking he paid a ton for not much. The extras don't add much to the total so why not be comfortable?
And I absolutely disagree with the notion that inexpensive and sporty = massive sales. Just compare the sales of 240Z's with ZX's. Bigger, heavier, more comfortable, more options = more sales and bigger mark ups (hence profit).