I beg to differ. Every published version of Ravenloft has had special setting rules to enforce the quirks of the setting, whether they be changes to the Turn Undead rules or the restrictions on Plane Shift and Gate to prevent characters from leaving. In the absence of a published 5e Ravenloft, it is up to the GM to come up with house rules to simulate the nature and feel of the setting. To me "you can't benefit from more than one short rest in an 8 hours period" is no more arbitrary than the RAW "you can's benefit from more than one long rest in a 24-hour period". If you want a story-base reason, it could be something as simple as "a pervading sense of dread makes it hard to relax; you are constantly catching hints of movement out of the corner of your eye and feel like someone is watching you, so as a result you aren't able to relax and rest as easily as you could in the normal world. When you try to sleep, your slumber is filled with nightmares which prevent you from receiving the full benefits of a long rest."
It's hard to project a feel of injuries being something you need to worry about and something that could be with you for a while when the base rules say that you could be beaten unconscious multiple times in a day and go to sleep with a single hit point, and when you wake up you are completely healed with no effects from the beating you received the day prior.