Zack Snyder's Justice League is no mere special edition or director's cut. Rather, it's an alt-sequel to the two preceding Superman films. The huge difference between the respective styles of Zack Snyder and Joss Whedon is very evident in this series; Whedon was not hired to mimic Snyder's style but to diverge from it in an attempt to correct (overcorrect) for complaints about its predecessor, Batman v Superman: Dawn of Justice*. They're even driving this point home for audience members who might not follow the finer points of film production, the marketing for the series is emphasizing that this is a different movie, which has the convenient benefit of being true.
The economics of streaming make this a lot more promising than a typical post-theatrical revision, too:
The first thing to consider is that WB owns HBO Max. They get all the revenue arising from this release, not just whatever pittance they'd get from releasing it on a competitor. The reason why everybody wants to run their own streaming service is because owning a streaming platform is a whole new level of profitability, driven by vertical integration. It might not be a winning proposition to release it on e.g. Netflix, because in that case it wouldn't be WB capturing the benefit of increased subscriber numbers over a period of months.
The second point is that streaming as a sector is focused on rapid growth. It's widely understood that there's only room in this pond for a few big fish, and HBO Max is well-positioned to be one of them. Aggressive plays for subscribers can pay off for real contenders the way they can't for wannabes like Quibi, and they're necessary to stay in the game. If ZSJL represents an overextension, it's the same caliber of overextension that's built into the strategies of every competitor in the field.
Thirdly, the format is very tactical here. Even though it was originally written as a film rather than a mini-series, they're releasing it as four episodes, spread out over the duration of a full billing cycle. (Changing the format certainly drove up the cost, as that sort of thing requires more reshoots.) That means a lot of the audience is going to stick around for the other programming on the service in between parts. At $70 million, it's still not even close the most expensive season of original content (though it is the most expensive per episode). It's well in line with other prestige content.
But, fourth, relative to other prestige streaming titles, there's already a lot more buzz around it built in. It may have only a vocal minority of superfans in comparison to a big-tent blockbuster (even that is debatable), but for a streaming show, having a vocal minority of superfans of any size is already a huge step up over most of their prospects. Even more so once you consider this is before release.
Basically, the disappointing return they got on Justice League (2017) was already accounted for in 2017. The biggest expenses involved have already been paid (the combined production and marketing budget of the prior film was literally ten times the amount we're talking about here). But a rare fluke means that there's far more opportunity left in the leftover materials of that production than is typical - a sufficiently new product can be made from the rough cut at relatively low cost, and distributed through a platform that will maximize its returns.
* (As a side note, the theatrical cut of BvS really is pretty weird and bad, especially in comparison to the Ultimate Edition. Merciless cuts for time even go so far as to eliminate establishing shots, giving the whole thing a sort of dreamlike pacing that's not really easy to follow. If you liked Man of Steel but didn't like its sequel, give the ultimate cut a try.)
