505 Games has written a statement explaining why only Control Ultimate Edition owners will get next-gen upgrades. You can read the statement in full here, or scroll below for a summary.
"We spent several months exploring all of our launch options for Control Ultimate Edition and no decision was taken lightly," the community post reads. "While it is challenging bringing any game to next gen platforms, we quickly realised it was even more difficult to upgrade our current user base to next gen with full parity across platforms with our year-old game."
Control Ultimate Edition comes with the base game, content updates, and all the DLC, including the latest Alan Wake tie-in expansion, Control: AWE. Only those who purchase the Ultimate Edition (launching first on current consoles and then PC before heading to next-gen) will get free upgrades for the PS5 and Xbox Series X. The reaction to this reveal was less than stellar from players who already own the base game and the season pass, but according to 505 Games, offering a free upgrade to all owners "meant that at least one group of players ended up being left out of the upgrade for various reasons."
It seems Remedy Games and 505 Games are working hard to develop Control Ultimate Edition for next-gen platforms and for new Control players. Considering the game is nearly a year old, the assumption that a free next-gen upgrade would come to all Control owners was a bit of wishful thinking. Fortunately, Control is backwards compatible with PS5 and Xbox Series X, so you will be able to play the game on new consoles and enjoy their processing power (my Xbox One is on the verge of exploding every time I play Control, so I need more power).
TL;DR: "To bring Control to next generation consoles in any form, we had to make some difficult decisions and those came in the form of creating the simplified Control Ultimate Edition product plan we have today."