Yeah, at that point, the only hardcover DS9 novel was KW Jeter’s Warped, which, at least in my opinion, was lackluster and feels like it got bumped up to hardcover during its writing, so the stakes abruptly jump to justify that. So the Avatar duology made more sense, not just as a DS9 event, but even in the publishing model of the time - when both novel slots of a month are given to the same series, the same event, that says something to the audience about this being A Thing™.
Between the success of the DS9 line that followed and the 10th anniversary, Unity as a hardcover made much more sense, but Avatar didn’t have that foundation going in.
I will agree that the first New Frontier books should have been bundled up - maybe as two books instead of four. I figure it may have been financially motivated - weren’t they also cheaper than the standard book at the time, so probably Pocket was hedging their bets in the name of a risk in the books being the first novel original line. But still, at that point in time, where the novels were all hitting around 270 pages, four books that don’t even crack 200 just seems particularly weird in comparison to the rest of the books published at that point in time.