China blocks WhatsApp users from sending photos and video

Photos and videos are harder to spy on.

However, this might be changing too...

https://www.wsj.com/articles/chinas-stopchat-censors-can-now...

Text can by monitored using automated tools, however the same capability is not as well developed for video/photos.

Also, pics/videos have more impact. If I describe the results of a beating it will not have as much of an impact as the photo of the broken bleeding person will have.

So for a government that is trying to control the narrative the ability to control the images being transferred becomes key.

You might be able to catch images/videos just by the fact that suddenly a lot more data is transferred. Or part of the metadata is not encrypted, uploads might happen over a secondary connection, ...

Maybe they are constantly interrupting the connection. Text messages would go through as the connection to the server to upload a text is for a very short duration whereas the image/video would require them to be connected for a longer time and the interruption would kill the upload.

(This is all conjecture obviously)