Just as Xia Qingqing and her party had guessed.
It would take time for stronger beings to descend from the Immortal World and the Heavenly Realm.
Firstly, they needed to report to the power structures in the Immortal World and the Heavenly Realm they belonged to. Many powerhouses cultivated in isolation for long periods. Deciding which of these to send required coordination.
Secondly, the spatial nodes connecting the Immortal World and the Heavenly Realm to Earth could not accommodate alot of power each time they were opened.
This led to a result: powerhouses who were too strong could not descend.
Level seven cultivators were the limit.
Of course, because the spiritual energy of the mortal world could not be compared with the Immortal World or the Heavenly Realm, descending from these realms was akin to an ordinary person going from a place with fresh air to a place severely polluted.