One thing people seem to be ignoring is theme. I know that back in the day, bands and producers spent a lot of time thinking about sequencing, particularly in the days of records, when sequencing was a big deal, having grand openers for side 1 and side 2, and having perhaps ballads, or melancholy tunes to round out side 1 and 2 or more thoughtful tunes. On top of that is the basic idea that some of these songs could easily not be cr@p, but rather could be songs that make much more sense as a part of a new album, then they do thematically with Sirens' Call.
Think of it this way. Would Low-Life be the same or make the same sense if Shellshock or State of the Nation were on it? Perhaps the germs of those songs found their origins in those sessions. Who knows? Thieves Like Us could probably fit on Low-Life without a problem, but Blue Monday could not. Similarly True Faith would never have made sense on Brotherhood, and Temptation, as much as we love it, didn't fit musically for PCL, even though it could have seen release as an early single for that album if New Order back then, was as laissez-faire about working on new material as they are today.
I think it could easily be as simple as New Order having record 18-20 new songs, and having done them, felt that these 10 or 11 felt right around one another. All the early write ups on the album seem to suggest a basic thematic quality to the album (other than the supposed glam/iggy and the stooges rave up that's on it), perhaps some of the songs did not fit that theme, and just as easily perhaps the band felt they didn't want to upset the delicate Technique-like balance of guitar and synth by adding certain songs that would have tilted the album too strongly in one direction or another.
This is all guesswork, but I do know that the reasons for exclusions of songs from albums can cover a heckuvalot more than filler/schlock/sh!t as justifications for leaving something out. Remember, the Stones, "Start Me Up," was recorded years before it ever saw release, whether they dropped it from their album of the time (I believe it was recorded initially in '77 and saw release in '81 if I remember right)because they simply didn't know what they had (just as they so severely underrated their performance in the Famous relatively newly released "Rolling Stone Rock and Roll Circus") or because it didn't make sense as a part of the album they were recording at the time, I do not know. However I do know that Start Me Up, when it finally saw release was certainly one of the best two or three songs the Stones had recorded in a decade, that it didn't make the album it was recorded for is a testament to the fact that sometimes great songs don't make the albums they were recorded for (I'll grant that some or most of you may find the Stones tired, or old hacks, or "Start Me Up," a tired standard rock song, but no one can deny it's staying power as a classic piece of rock and roll history).