Because it is hard to develop the brand equity to sell cars in the $70k-$140k segment and the fat margins they command. Ask Cadillac,Lexus,Infiniti,Acura and Lincoln how easy that is to do. Even Hyundai is trying to do this with very limited success. Taking the "Model S" brand down market would be incredibly stupid. It leaves profits on the table that can be used to fund superchargers,stores, service centers, R&D and Gigafactories to go downmarket.
Model 3 will sell in the $35k-$70K range and maybe overlap a bit with Model S.
Perhaps in the not too distant future Tesla will have a small medium large lineup like 3 Series-C Class-A4, 5 Series-E-Class-A6, 7 Series-S Class- A8.
Absolutely true.
The Tesla Model S is a car in the F-segment (large, luxery and sports). The growth of the global marketshare for the Tesla Model S will not only come from it's competitors in the F-segment, but besides that also from other segments too (and certainly in the next two years).
The point is that people now have an option which wasn't available before. And as the global Tesla Supercharger network is expanding more and more, this new option is becoming more and more attracting to more and more people.
Actually, a whole new segment has already been created (Tesla Model S + Supercharger network), but it has not been valued as such as of yet by most of the people. But that will come in the next few years.
You see, other brands don't offer a value propesition like Tesla Motors does. They don't sell cars with the promise of free fuel for live (for long distances). When people will start doing some math calculations, they will be surprised and they will start scratching their heads.