Easy. You don't write a major motion picture script hoping
you can coerce your anticipated main guest star to participate. Since TNG fans would only accept one actor as Q, John de Lancie's agent would have Paramount over a barrel as regards salary.
Paramount: "We want you in the next movie."
Agent: "We need to see the script."
Paramount: "It's not written yet."
Agent: "Get back to us when you have a script."
(Major expense needed, for no guarantees.)
Agent: "We hate it. John's passing."
Agent: "Yes, it'll do. Our price is extremely high, you know."
Agent: "Yes, it'll do. We are very busy, though. Call us in maybe a year or two."
Harve Bennett didn't have this problem with ST II because that script wasn't written in the hope that Ricardo Montalban was available or interested.