Jellly,
Good Question.
Maybe there are answers out there but this is what I know and have come to understand from a fish monger , Mary :
"There are many reasons for why a crabs meat taste sweet. Mostly it's due to the waters it's caught in. But it can also be due to whether or not it's ready to molt, has already molted but the meat hasn't yet developed in the new shell, how it's processed, and whether there is crab fat in with the meat or if it's been washed and is very white. Also what they eat is of importance as well."
Hope this helps,
Petals.