Hmm? Isn't that logic backwards though? You're making it sound like a port requires practically any resources at all and in fact it seems like, with the way you worded that, you're almost making it sound like a port would require more effort than an expansion. That's obviously not true though--it's ports that require minimal effort and expansions that actually require using resources. Why bother "wasting resources" on making expansions, when they could instead save that money an resources, make more ports instead, and still be raking in the profits?
When either way will work, it would seem more logical to take the one that requires less resources to me. And in any case, since ports are minimal-effort endeavors, it's highly unlikely that another port of Portable 3rd would take away any actually significant amount of resources from a theoretical team that's developing a Portable 3d expansion anyway (or in other words, there's no reason both can't be done simultaneously), so it's not a problem to begin with.