I don't think it can be that difficult, it's in 70m but that's 30m less than Britannic. Certainly not a beginner's dive but almost routine for a tech diver. I suspect the reason it claims so many divers is because there's so many divers on it and the laws of averages claim the right number. Put the same volume of traffic on Britannic and it would push up the price of body bags.Andrea Doria is heralded as a difficult wreck to dive, I am told it is A bit of a Mecca for Tec divers.
There's a good book about diving the Doria called Deep Descent and another about the sinking called Colision Course if anyone is interested.