Does anyone know why Ryainair tickets cannot be imported into CheckMyTrip?
You can only import bookings to CheckMyTrip, which were made through Amadeus GDS (since it's a system provided by them).
They also mention this in their FAQ:
Only bookings done on the Amadeus booking system (for travel providers that are customers of Amadeus solutions) can be imported through the manual import feature. Some of these providers do not wish to display their bookings on CheckMyTrip automatically. You can forward your booking confirmation email to email@example.com from the email address linked to your account and the details of your trip will be automatically added in your trip list.
Ryanair however has ended their partnership with Amadeus end of 2017: https://corporate.ryanair.com/news/ryanair-to-end-gds-partnership-with-amadeus/
When you book with Ryanair directly, they don't use a GDS (using it would cost fees, which Ryanair wants to avoid obviously). If you book a Ryanair ticket through an agent, then it's most likely booked on either Sabre or Travelport.
The CheckMyTrip webpage FAQ under the header "Trip Import" says that only travel booked via the Amadeus system will import. Perhaps Ryanair uses another system, or refuses to allow their data to appear.
Here's a screenshot of the text: