Pretty much this, plus the fact that RVLoader (the software that makes Wii portables possible) doesn't support disc loading and in fact patches out the disc drive calls. To incorporate the disc drive into a portable basically eliminates any space, weight, and power consumption reduction entirely and no-one does it outside of the occasional Laptop type builds.
It is possible, but there is essentially nothing to gain from doing so. It's a lot more work, and the end result is a portable that's much larger, has horrible battery life, and is far less convenient.