Because it is designed to be deflationary?
Because its (energy cost) overhead is still sky high and likely to remain reflected in prices?
Because it is the "brand name" market with whatever caché remains to that name reflecting in prices?
Because sunk costs of existing investors want to keep the price high for as long as possible?
Those are just a few of the more obvious reasons. Not that I think it's any one thing here, it's probably some admixture of the above and other reasons.
This is a huge fallacy.
https://digiconomist.net/bitcoin-energy-consumption
https://arstechnica.com/tech-policy/2017/12/bitcoins-insane-...
Yes, it's down from the bubble peak in mining, but it is still a massive over-expenditure of energy per value. Right now, still, according to the Digiconomist stats above, ~15 average US homes can be powered for a day with the energy spent to make a single Bitcoin transaction block. That's a considerable amount of energy.
The mining viability follows the price of bitcoin, not the other way around. The mining costs are in no way reflected in the costs of mining.
That's because bitcoins base price is a very round zero, and the only value associated with them was derived purely from speculation and prescribed prices used in money laundering operations.
[1] https://medium.com/@PanteraCapital/market-update-pantera-blo...
[2] Crossing the Chasm, 3rd Edition: Marketing and Selling Disruptive Products to Mainstream Customers. Paperback – January 28, 2014
[3] https://www.investopedia.com/articles/investing/091814/what-...
https://www.investinblockchain.com/cryptocurrency-adoption-r...
BTW, the reference states
"The study indicated user growth rates were higher in 2017 but continued on into 2018 at a rapid pace."