Unfortunately I cannot answer that as I've never used BitLocker myself. The fact that it works out of the box in other distributions makes me think ZorinOS is not using any compatibility for this feature in particular.
Just real quick I'd like to make clear that the goal of ZorinOS is not to become a feature-by-feature clone of Windows or MacOS. It's goal is to provide an alternative that reassembles those environments to make as smooth a transition as possible. Still, it's a different operating system and there will be fundamental differences that may or may not be overcome easily.
As mentioned in a previous post, Bitlocker is a Windows-specific tool. There may be attempts to bring the functionality to other operating systems, using tools like dislocker, which is probably what other distributions include by default.
Why this hasn't been implemented in ZorinOS is up for debate. I would guess it's just not something many people use or have replaced with Linux-specific alternatives. However there are others who have reported similar issues recently, so it may very well be addressed in future versions of ZorinOS.