Soldato
If nvidia was to sell both at the same time it would have to sell the 3000 series at a much lower price than the 4000 series (especially if the 4000 series is significantly more powerful, as it's expected to be). Which would mean either selling the 3000 series at the same price it is now and pricing the 4000 series much higher or selling the 3000 series at much lower prices than it is now. I don't think either would work well for nvidia because the 3000 series cards would still be good enough to result in many people choosing them over a much more expensive 4000 series card. Maybe they'll rebrand 3000 series cards as lower model number 4000 series cards, maybe with a minor change or two so they can pretend it's not just a rebrand.
The reasoning behind it I believe is supply. Stocks of 4xxx cards are going to be small. Whereas stocks of 3xxx cards will still be in fairly good quantities now AIB's etc have caught up a bit.