Because such links do not refer to the image file.
The format of the link appears to be OK. It should work for miniBB (I've tried it - the image BB code allows this format).
But the URL itself opens an HTML page, not a binary file.
This is the right URL for the image you mention:
But it won't work for miniBB without modifying its default BB code... you have to insecure miniBB's BB code to allow such images to be inserted.
If Dropbox would have a sharing option, I could program BB code for it.
But it appears, they don't have it at all... so they don't want their images to be shared?
Then why should we take care of it ;)