It's actually easy to explain: all luxury manufacturers offer (at a hefty price) high quality interiors - model S does not. After 2 years in my 125k car, I want to be able to get better interior materials, and I am willing to pay for it. I bet I am not the only one. If you compare the quality of materials in highly optioned MB / Audi / BMW you will now what I mean. And no, it is not about stiches on the seats and fluff, but rather the beauty of the impeccable fit and finish. If they had one on model S, I would have ordered it.
Getting back to model 3, I was thinking about 10-20k for upgraded interior, add another 30k for largest battery and AWD. That puts the cost of what I want to purchase at ~75-85k, which I thought was reasonable - and is ~75% of the 100D with the options I want (which ticks in at 115k). I can afford either of them, but I do not need a car as large as model S. I even like the minimalist design of model 3 - provided it is executed well. If Tesla is going to compete with the big boys in the entry luxury market, better shape up.