Because let's say I have a lightning tuning for sale. I don't want to sell it for 15 million credits. I think it's worth much more than that. So what do I do? I go to trade chat on the fleet and offer it instead of on the GTN. I KNOW someone will pay more than 15 million credits for it, so why not?
If you then decide to head that off by capping credit trades between players at 15 million too, I'll start offering the tuning only in exchange for something of equal value (in this case, the tunings regularly sell for 40 million). I'll want a Tulak Hord lower body set, or a different tuning, or a couple hypercrates, etc.
Your idea makes it even harder for YOU to buy these things.