What he said..
As far as why... The events in a-spec are already programmed in. The events were created already with specific tracks. People have already completed these events and earned trophies so they can't change them now.
I would like to to see them add new events to A-spec but for the reason above they cant. I assume this is why they are adding new events to the seasonal event section. I'd love to see them add the new tracks as seasonal events but I don't think they will here either because if they added the new tracks then people who don't buy the DLC would see the events and not be able to participate. Another option would be to push an update to only those who purchased DLC so only they would see the new events. But then PD would have two version of the game out there which would make supporting the product much more difficult.
So you can race the AI in arcade mode, or you can do time trials in practice mode or one make races in practice mode. But online will be where it's at.
downloaded new content but the new tracks are not in a-spec WHY NOT?
There is no Aspec event using the new tracks yet. Be patient.