It creates a natural pacing nightmare that could only be solved through either massive cuts in the story (which nobody would want), or the making of an adaptation with a more expansive runtime.
The project could have tried to go the route of Andy Muschietti’s IT – which is to say split the source material in half and hope that the first movie would be popular enough to allow the second to get the green light – but making it as a Netflix show makes a great deal more sense.
Properly getting across that much exposition and giving introductions to the mechanics of both worlds in The Talisman would alone likely take at least 30 minutes to explain cinematically, and that’s a hell of a lot of real estate when you’re talking about a feature film that is meant to also showcase a pair of cross-country journeys.