That would only make sense if you a) had a competing product on the market or b) at the very least actually announced one. The remaster/remake is a mere rumour, we don't even know if there's substance behind it.
Opportunity cost, possibly. They might be remastering it and wish to sell it at a higher cost, or think they can get more money through a different retailer.
That game seems pretty loved, I wouldn't be that surprised about a remaster. Though it would have to be a pretty barebones if it can't compete with a ten year old release.
In order to get fully set up, you will need to pay a $100.00 fee for each product you wish to distribute on Steam (the "Steam Direct Fee").... This fee is not refundable, but will be recoupable in the payment made after your product has at least $1,000.00 Adjusted Gross Revenue for Steam Store and in-app purchases. Steamworks Partner Program
While not free, seems to be no overhead for the length of stay on Steam that I can see. Another site includes some more description which might add to the cost though
The cost to put a game on Steam depends on the type of game, the development budget [1], and the business model. To get a game onto Steam, developers need to pay a one-time registration fee of $100. After that, they will need to pay a 30% royalty fee. Additionally, developers may need to pay for marketing and other costs associated with the release of their game. source
Regardless, it's just another unfortunate case of physical copies/backups>subscriptions or "licensing/rent" deals.