What is the appropriate way to request a creator to add more of their designs to MOCHUB?
Some creators may not want to share building instructions as there is a fear of knock-off companies releasing them as sets. Especially if MOC's are of top quality.

It is also possible there a personal choice to limit (control) their work results via fewer platforms. Or simply may not like some platforms at all.

Edit: Thinking from my point of view (someone who has created and shared instructions for LEGO MOC) I would consider alternative web redources if there would be obvious benefits. So it might be worth finding such reasons, like audience growth, unique features, something else. Then contact the author via one of the ways he is available and suggest him to contribute to your choice of web portal listing the possible reasons. It could be the easiest way of letting this person know and understand if he has anything against it.