This is exactly how Nintendo views VC. As filler. They also believe drip feeding releases leads to more sales.My only guess is a VC release is seen as something to fill the gaps but at the same time it is popular enough to threaten the rest of the shop. If you're an Indie developer the last thing you want to see is your game sabotaged because DKC came out on top of it.
Supposedly there's a gigantic VC backlog of finished titles waiting for release going by comments from Capcom and Natsume reps. Nintendo also still sets pricing and dates for VC, unlike all other eShop games. The only likely exceptions are 3rd party platforms which presumably Sega (Game Gear) and Konami (MSX, TG16) control instead.
NOA probably hasn't given us the DK trilogies yet because we got Captain Toad for the holidays. DKL was rated by the ESRB almost a month ago so they're obviously just stuck in the que waiting.