If I had to make an honest guess about the Metroid Prime Trilogy thing, it would be space issues on the disc.
The original Metroid Prime was about 1.2 GB, barely fitting on the GameCube disc. Metroid Prime 2 was about the same size. Metroid Prime 3 was about 4.4 GB. In total, that's about 6.8 GB. If you assume that the first two games need to be adjusted to account for being on a new platform with a different native resolution and may also have a new control scheme, it may be easier to just write new data on top of the old games instead of completely rewriting the existing code. So between that and the few new things included in the Trilogy, that could be another 1 GB. That's 7.8 GB in total.
The standard Wii disc only held 4.7 GB of data.
In order to fit the entire trilogy on a single disc, Retro must have used a double-layer disc, something that the Wii probably isn't designed to work well with. And even then, I don't think dual-layer DVD's can actually hold twice as much info, only about 70% more or something.
So 4.7 times 1.7 is about 8 GB... barely enough room to contain the entire trilogy.
tl;dr The Wii hardware wasn't the problem, disc size probably was.