DKCR alone sold more than the complete Prime Trilogy combined.
DKCR3D alone outsold mp2 and mp3.
it's easy to see why Retro did DKCTF.
Donkey Kong Country Returns sold a little over 5 million copies worldwide I think.
Metroid Prime sold around 2 million (such a shame)
Metroid Prime 2 sold less than a million, 800K at last count (even more of a shame)
Metroid Prime 3 sold about 1.31 million WW at last count (better than 2, but still a damn shame)
There are no numbers for the Prime Trilogy edition, but since Nintendo only had it on store shelves for a few months it's probably for the best.
When you look at the numbers it really is a damn shame the series didn't sell better. It's just an amazing compilation of fun and immersive gaming that more people should experience.
I absolutely cannot fault Nintendo/Retro for continuing on with DKCRTF after the sales of DKCR (and am glad since TF is an even better game, and at the very least it gave us godly David Wise music again). And I am just as sad to hear that game only seems to have sold a few hundred thousand copies. But again that is probably more to do with the Wii U.
I suppose that perhaps TF's "failure" may possibly give Nintendo the incentive to have Retro either go back to Metroid or try something new.