They have to hold off the announcement so that they don’t spoil some of the ending of Picard season 3… probably. Also the Networks and Producers probably have not made any final decisions as of yet in regards to potential spin-offs.
Obviously the Enterprise F is in Picard season 3, this ship will have a Captain. If we know through the announcement of a spin-off that Seven will become the Captain of the Enterprise then we know that its current Captain is not going to last past season 3 of Picard. That could potentially be a *major* spoiler.
Assuming there is a Seven spin-off in the works and further assuming that P+ decides to start the hype train before PIC S3 has wrapped, it's easy enough to drop a spoiler-free teaser.