The most appealing route for me is to assume the reason there are Wolf 359 assimilees in Delta is related to
1) why the Borg spent so much time at Wolf 359 to begin with
2) why the Borg left so many starships basically intact at Wolf 359
3) why the Borg bothered to capture and keep captive even the old tub
Saratoga, and
4) why there are no lifesigns around when the E-D arrives.
All we need to tie this all together is to remember ENT "Regeneration". Given enough time, the Borg will turn any pile of junk into a Borg Cube. Indeed, quite possibly this is how all Borg Cubes begin. So the E-D caught up with the big Cube chiefly because it had supervised the construction of many little Cubes out of Starfleet hulks, and their crewing with carefully assimilated Starfleet personnel.
Really, though, they should've encountered more Alpha/Beta Quadrant ships in the early years, because we know that the Caretaker abducted a bunch of other ships from all over the galaxy before abducting Voyager.
But we don't know that the Caretaker would have released any of those. The two Starfleet ships struggled free, one because the Caretaker as already dying; there was a whole junkyard of ships that didn't make it, though, plus then ships the Caretaker might have sent back (he had the ability, although we don't know if any return attempt was ever made, or if any ever made it back). Neelix seemed pretty well informed, and he only knew of about fifty abducted ships; the
Equinox and the Dreadnought managing to escape could well be the full extent of exemptions to the rule (unless the Caretaker captured more crewless spacecraft in error - he'd have released all of those, of course).
Timo Saloniemi