The seven seals identify God as the author of the scroll, not John. So nobody in the created world (heaven or earth or under the earth) can open it. The event of the Cross, which makes the Lamb worthy, comes out of the relation between the Lamb and the one sitting on the throne. We know from John ch1 v3 ("All things were made through him") that the Son is not part of the creation but sits on the "Creator" side of the boundary between Creator and created things. He is now in heaven having returned to his Father in the Ascension. (This half of the answer was given previously).
The second half of the answer. What is unique about the Son, the Lamb, is his role in the Atonement. The Atonement is the driving force behind the events of Revelation. It is the Atonement, as we learn in ch12, that drives the Accuser "out of heaven", out of his place of power over men; "They have conquered him by the blood of the Lamb" (v11). We learn from the second half of that chapter that the Atonement has provoked the anger of Satan enough that he devises the great persecution as a way of getting his revenge. In effect, "If I can't accuse them before God, I will accuse them before human authority instead".
And we learn in ch5 v9 that the Lamb is worthy to open the scroll "for thou wast slain and by thy blood didst ransom men for God". The events contained in the scroll are the means by which God "rescues" his church from the persecutors, and that is what makes them valuable. The implication is that the Atonement, which saves us from our ultimate enemy (sin-and-death) is the source also of his power to deal with our lesser human enemies.
That is why only the Lamb can open the scroll. It is his scroll, just as much as it is his Father's scroll.