Because in EU IV playing as a trade power means that your diplomacy & conquests revolves around trade. Not so much in EU III. For example (pre 1.8 patch) as Ragusa I crushed Venice but didn't wanted their land so I had them transfer their tradepower to me. Then I moved all trade to Venice node & collected there for a couple of years. Then I thought about moving even more trade to Ragusa so I carefully invaded Mamlukes & conquered Alexendaria & built my base there & strengthened my tradepower in that node. This gives more flavour to the game & you can roleplay it in more interesting ways. If it was EU III I would just spam merchants in every nearby CoT regardless of their location with almost no strategy & too much micro. Here I had to selectively conquer stuff & make sure not to annoy Ottomans or Austria (although I ended up having a fight with Ottomans because they wanted Egypt for different set of reasons than me).