As Marc-Oliver Frisch points out, when the first two issues of a series appear in the same calendar month (like "Superman/Supergirl: Maelstrom" or "Black Lightning: Year One"), the drop off in order numbers between issue #1 and issue #2 is much less.
Retailers order far more copies of the second issue, proportionally, than they do if there's merely a one month gap between issues.
If that's true, and it seems to be, then why do Marvel and DC even bother with monthly (or, worse, bi-monthly) miniseries? Why not just wait an extra few months until the whole series is in the can and release all 4-6 issue miniseries as weeklies? The sales boost across the line would be pretty significant.
And I don't think it's just a matter of tricking retailers into ordering more. I think readers would tend to stick with even a mediocre comic for more issues if it came out on a weekly basis. See "Trinity" for more information.
Not that I'm promoting mediocrity, but I'm just wondering why publishers wouldn't take advantage of the retailer/buyer patterns.