This is most likely an "let's just do it all" move since they'd want to end Mario Kart Wii prior to Mario Kart 8 coming out. It may be required to be all-or-nothing to take Mario Kart Wii out, but that's just speculation.
Its bad speculation IMO. The Wii Mini entered the scene over a year ago in Canada I believe. It makes perfect sense now if they knew they were going to axe the online once the new generation got underway. Im much more likely to trust 10+ years of evidence that says Nintendo just doesn't care about online than I am to assume they are competent enough to axe online for the sole purpose of possibly adding sales to a single title on their current dying console.
They let MKDS reign on for nearly 10 years. I don't think they mind having more than one MK game on sale at a time.