"which for most people is much more worthwhile."
It's a matter of bits and pieces, each one of which creates an incentive/disincentive to purchase the console. On my end, if I buy something that wants to replace the Wii, I want it to do EVERYTHING that my Wii does so I don't have to shove another unit into my gaming cabinet, and I have far more GameCube games than I do Wii games. Titles like Twin Snakes, Eternal Darkness, Second Sight, Smash Bros Melee, StarFox Assault, Crystal Chronicles, REMake 0, ReMake 1, MegaMan AC, GC Twilight Princess, the Zelda AC and the original Wind Waker still look great to me and get regular play on my system.
By removing GameCube BC, it's a chip in a long list of cracks the Wii U possesses that create a disincentive for purchase.
Do I enjoy the few Wii-specific games I have? Of course. I own XenoBlade, Last Story, RE4: Wii Edition, Silent Hill Shattered Memories and the Prime Trilogy Collectors Edition. But that small list of Wii titles pales in comparison to the full library of games existent in my gaming cabinet. The list of Wii U specific titles that are worth buying is so much smaller still, that to be perfectly honest buying the console would be in essence paying $300 for a less capable console than I have now.
You can say to yourself "Well, I don't use this, so it doesn't matter"- but if there's one example of someone who isn't buying the system for any particular reason, it stands to reason that there are many, many more.