Simple: because Nvidia managed to improve efficiency even further.
At the same node it's slightly more powerful and slightly less power hungry. So the emitted heat had to drop significantly. And this is the most important gain here.
This means 1660Ti is a chip that can be paired with a small cooler. In fact most companies that announced their lineup have a compact ~18m variant. And the resulting cards are cool and quiet.
Some companies tried their luck with compact 1070 cards, but with mixed success (and none was as quiet as this one).
Think about what this implies for mobile variants.
As for the price: you can't expect Nvidia to adjust their pricing to the deals we get from stores for the older product. It can't work that way.
1660Ti card launched at $280, so based on MSRP it's actually way closer to the 1060 6GB ($250) than to 1070 ($380). A theoretical non-Ti 1660 would cost as much as 1060 did. So yes, 1660 is replacing 1060. And yes, it's faster and more efficient than 1070 - a card from a higher segment.
As I said: current RTX implementation and utilization far from what is possible. But it's not a reason to criticize the technology. We'll slowly get there but we need time and chip performance.
Remember RTRT is not a gimmick. It's not something Nvidia created as just another feature. It's not something we could backtrack from because we don't like current results.
Ray Tracing is how photorealistic renders are made. And since we got into 3D gaming, we knew this is how games are going to be rendered in the future.
Now, we could stay on the curve of GPGPU potential and get gaming cards able to do RTRT in 10 years. Or we can utilize purpose-built RT ASIC and get this tech now.
And once again: RTRT doesn't mean games will look more pleasing. The exact opposite is more likely.
