The speed of cars mostly comes down to how much power they have. The main reason for having more power is not to increase top speed, which is limited by law, but to increase acceleration, which is not.
And now a days, bragging rights. People like speed - period. If 1 model out performs another model at a similar price tag and features / design - people will go with the faster one (usually). Same as any other personal preference - people shop with their eyes first, wallets second. If they like speed, that sells more cars.