The main reason is the lack of hauling capacity.
After World War Two a good number of cargo ships were dumped on the surplus market and used until they gave out.
They were often employed as tramp freighters, which is the basis of the Free Trader from Traveller.
The Victory ship is not regarded as a particularly large cargo ship. It can still haul a half million cubic feet of cargo, which is 1,000 dTon in Traveller terms. But it can only carry 4,488 sTons of cargo (most of the deadweight tonnage is fuel and ballast) weight, which would be less than the 5 sTon per dTon that's standard for Traveller. In theory one can use cargo as ballast, which raises the cargo weight to 7,617 sTons.
The 200 ton Free Trader is the moral equivalent to a Victory and it's just 60.5 dTons of cargo space. With 10,000 sTons of contragrav it can carry more weight (9,634 sTon), but it'd have to be very dense.
One thing that's nice to see, the 5 sTon per dTon is completely reasonable by WW2 shipping standards!
Good numbers on WW1 and earlier cargo ships is thin on the ground. It's been put forward that the Free Trader is actually an Elizabethan cargo ship because the travel times are similar. I don't have anything on what one of those could carry.
Traveller Tuesday officially belongs to Erin, I'm just dangling from her coat tails on the idea.