This is because other channels you imported from Kolibri Studio do not contain exercises that are needed to create quizzes.
If you go to Kolibri Studio website (https://studio.learningequality.org), you can check the types of content the public channel contains by clicking the name of the channel.
this image shows the types of content that
Be strong; Internet safety resources contains:
and here are the types of content that
EngageNY (en) contains:
Please let me know if you have any questions! Thank you!