It's not exactly a market I'd want to ignore. "Let's ignore this rapidly expanding, highly profitable market in favor of our slowly contracting, less profitable one."
The fact is, home consoles are not showing a sign of contraction, if we consider worldwide sales. PS4 is the fastest-selling home console ever, and Xbox One is not doing badly in the US and UK. The traditional market is also a lot less predictable than the mobile one. Nintendo might succeed with some IPs, but a. it's not automatic (we have seen traditional companies struggling on mobile, such as Capcom), b. it might not a good long-term investment (mobile market is less stable and people are ready to jump on the next big thing quite often), c. fidelization is something that on mobile is hard to achieve, unlike dedicated devices.
If Nintendo has this variety of IPs and characters it's because it was able to fidelize consumers. Mobile market has yet to see a success story in this sense. Angry Birds is the most important mobile IP and it's fading quite quickly.