I don't see the difference
You could post the same content on Twitter as Imgur, so why not allow both.
"If you can post same content, why change?"
Its easier to link something then having to go on twitter and upload the image
No support.
Twitter has better 'filtering' than lmgur. You could post anything you want on lmgur.
^ ROBLOX's actual reasoning (they've said so, I believe it was in the blogpost saying they're allowing twitter and youtube links only).