Because the entirety of the shale gas/oil boom in recent years has been 49% hype, 49% stock market swindle, and 2% oil and gas. That's why, as an industry, it seems immune to market pressures that should theoretically stop it in its tracks.
With lift costs in the $80-$100/barrel range, the industry was always running on the thinnest of margins, even at the best of times. The stock-market capitalization of the fracking sector has ballooned exponentially beyond anything that could be justified by the underlying profits.
Putting the brakes on development would draw too much attention to the fact that the industry was fundamentally doomed by diminishing returns from the beginning. Investors would stampede the exits and billions in shareholder value would disappear overnight.
With a little luck, the stock-market gamers behind this success story can keep the balloon afloat a little longer...