Except the Elves are probably the most powerful nation on the planet, barring the lizardmen who don't really seem to leave Lustria very often. Hell, part of the reason Marienburg is considered the premier port and its houses the best merchants of the old world is that the elves trade there. And if they want to put pressure on someone why wouldn't trading directly with that person help them more? That way they could threaten it themselves instead of needing Marienburg to do the exact same thing. Also, Ulthuan isn't exactly in need of goods, so wouldn't mire markets be better, both for information gathering, and for reaching untapped markets?
Ulthuan seems like they'd consider trade merely a means to an end, and that end is the iron grip they have on Marienburg via the 'Continental Exarchate of the High Kingdom of Ulthuan'.