Ann: Feasibility Study Lowers Costs & Increases Financial Returns, page-16

The first reply sums up the reasons quite nicely and the third/fourth replies provide some nice example data too, if you'd like to hear a more exhaustive explanation, I strongly recommend the very recent Crux interview with the Boss Energy CEO, easy to find on youtube. They're actually not interested in restarting at all until significant increases in spot/contract pricing, even though as you say they could restart now for a tidy 24 cents per pound selling into the spot market (lol).

Also, I notice your post history doesn't have much/any uranium discussion so possibly you're not aware and are approaching this from the perspective of a gold miner or something along those lines. Uranium is currently predicted by many to be on the way to a huge supply deficit/squeeze, as such there is a large amount of interest even in developers who cannot produce profitably at less than double the current spot price (!) (though as noted, long term contract prices are often MUCH higher than spot, e.g. PEN's ongoing contracts are at $50+). It's a super interesting and unique market, DYOR and I promise you'll have some fascinating reading ahead of you. One of my favourites I read when I was first researching it was a blog post "Would you have made a fortune in the uranium bull market?" about the previous supply crunch, very nice non-frothy article.

FWIW, BOE is one of the more promising tickers on the ASX, hence being valued higher than all but PDN; their mine is fully permitted and has produced before, whereas a new mine can take as long as a decade to get up and running by which time the bull market would most likely be over. Certainty of near term production is a rare and extremely valuable quality for those who believe the spot price might double or quadruple over the course of the next year or two.