Quality games that stood the test of time.
The amount of people who still go back to SNES, N64, GC and Wii games is a testament to that. The focus on tight gameplay and evergreen titles has kept people engaged, till now Nintendo games are the ones that dominate the speedrun scene.
I could go on about this, but simply I have returned to Nintendo many times purely because of nostalgia. I can go back and play Mario 64, Windwaker or Wii Sports and get immense value from it. They have rarely followed trends (making a shooter in the CoD era, or going for flashy graphics), and over time I think their games just stick out among look-alike games.
They kept those games exclusive for their hardware, and I think that association of needing to buy a Nintendo console to play a Nintendo game is inherently engrained in the customers / fans. There are obviously other factors but I think that's the main one







