They might not be responding because they have anxiety.

Or

They might need to think over their response.

Or

They might not have the bandwidth to handle dealing with your text at that moment, but wanted to check to make sure the text wasn’t urgent.

I can take days to respond to messages because I’m not mentally up for it, however, I don’t want to leave someone hanging if it’s urgent. Throwing in the “it’s rude” just makes it even more anxiety provoking and makes it less likely people will respond.