Its more of an agreement with the officials. Even at Rungis most wholesellers only get fish that was frozen. (From big trawlers with long routes) It is also cheaper because they can handle it longer and thus speculate on prices.

EU rules come in and say that even fish that was caught 200km away and thus takes a quality hit from freezing has to be frozen as soon as possible to prevent parasite problems.

Now i have special dealers who buy from small quick Fischerman and sells the same day to me (8 hour drive of a middleman with coolwagon) so i am the first one in the chain that has to freeze it.
Now my permission by local offiials is to serve it unfrozen if i inform my customers. Which is very nice of them, because EU regulation sucks the balls out of everything high Quality imho