That doesn't explain why they needed the Ent-E's deflector to send a signal. Plus the Vinculum thing they invented for voyager that connects all the ships to the collective.
Probably because the deflector was the only thing on a Federation ship capable of channeling massive amounts of power (which would likely be needed for a Borg transmission to actually reach the Delta Quadrant in any appreciable amount of time and convey a message without much/any degradation).
In Regeneration, the Borg who assimilated the Transport ship had to work with 22nd century technology (slowly transforming and upgrading it - which it did) but otherwise had 24th century knowledge.
Its part of the reason they were able to even send off a transmission to the Delta Quadrant which also degraded.
Subspace communications are much faster than that in the 24th century off the bat, but the transport ship wasn't still upgraded to match 24th century... most of its baseline was 22nd century and new systems were being created
The Vinculum brings order to chaos on a Borg cube... I'd imagine that on the Enterprise-E, the Queen essentially served that function (which IS technically one of her functions) - hence, there would be no need for a Vinculum there (plus, the Borg haven't had a chance to spread through the rest of the ship and didn't have computer control, so, a Vinculum wouldn't have been needed as of yet, or at all for as long as the Queen was there).