Why can't I save CanvasRenderTarget2D objects in a savegame?

Hi, bit of a frustrating issue i’m facing.

I have a game - a Q+A type game - which builds it’s assets on the fly from a web-page (BLUI) which is dissected into image tiles.

I have a struct array which holds all of my question data which is -

QuestionID (string)
QuestionInfo (string)
Some more strings (strings)
Another struct which usually contains 9 ‘tile’ objects (TileStruct)

The (tileStruct) contains:
AnswerImage(CanvasRenderTarget2d). <<< the problem.

When I populate the struct and use it it works fine, when I try to save the struct in a save file though only the very basic variables like strings, ints, floats etc are stored. I can recall this data when I load the savefile, but the savefile doesn’t seem to save the image data in the CanvasRenderTarget2d elements.

When I look at the save file it’s only 280Kb - so i know it’s not saving the images.

Thanks for any help you can give!