1: Nintendo portable systems have sold really well, so the market could be large. DS sold 154 millions units, 3DS has sold 65 million units so far. Of course no guarantee Switch will approach that. It is more risky with the hybrid approach, and continuing stiff competition from smartphones and tablets.
2: Handheld/portable systems are often better fits for indie scale games. Smaller/cheaper/shorter games.
3: Less AAA competition. Nintendo systems haven't been getting the biggest, latest greatest AAA titles from the large publishers, so indie games have more room to stand out.