Nintendos first party offerings are so strong that customers can satisfy their gaming needs just on Nintendos products.
The main reason I buy 3rd party games on my switch is the convenience of having them on the go and if they are very cheep. Indie games work well in this regard since they often cost less and have a certain uniqueness about them that can spark interest when browsing the shop.
Generally, offer better deals/products than the competition. Many third party games runs better on other hardware making them a harder sell on Nintendo systems. If it is the only system one own they still need to beat the first party offerings.