Why didn't Jor-El warn the council earlier that Krypton was doomed?
The film transcript would strongly imply that he had spoken to them about it before, at sufficient length for the Council to spend their time looking at each other and rolling their eyes during his latest speech.
Jor-El: We may only have a matter of weeks. I warned you. Harvesting the core was suicide. It has accelerated the process of implosion.
It would seem that Jor-El was ignored, and continues to be ignored because his tedious doom-mongering speeches about how they're going to 'kill the world' are long on apocalyptic warnings and very short on actual solutions to the energy crisis. His only suggestion is to reactivate the lost colonies (dumb) and to try to evacuate as many people as possible (basically a recipe for civil unrest, riots and the end of civilisation as they know it).
The film's official novelisation is in broad agreement.
Council Member Lor-Em, seated to the right of Ro-Zar, waved away Jor-El’s impassioned declarations.
“Our energy reserves were exhausted,” he replied. “What would you have us do?” “Reach out to the stars... like our ancestors did.” Jor-El tried to get through to the Council members, all of whom had inherited their positions by virtue of genetic heritage. Like too many Kryptonians, they seemed more concerned with preserving the status quo—and their comfortable lifestyles—than worrying about the future. “There are other habitable worlds within reach. We can use the old outposts—”
“Are you seriously suggesting we evacuate the entire planet?” Ro-Zar scoffed at the notion.