4k resolution is not mainstream yet. 4k also requires fast driver circuits (read pricey) to display the HDMI/DP you plug in. 60Hz is the de facto refresh rate for monitors, no ones gonna buy it if you sell monitors with 30Hz.
There are few manufacturers who have 4k at higher refresh rates (Asus for example). But, you have to pay a premium price since the buyer pool is limited(You have to pay for their R&D too).
If you can buy 2x 1080Ti to play at high refresh rates in 4k, why not buy a 4k 100Hz monitor for the price of one 1080Ti?
Once 4K becomes mainstream, more and more people start going for 4k, mass manufacturing and divided R&D costs will make the 4K cost less. Then you can see the 4K 144Hz come into market at a lower price.