The answer is simple. They feel like the Switch is a winner. While it may not be all that powerful, it can still run all the modern game engines that developers like. Games that work on Xbox One and PS4 today can still work on the Switch and it will be a good investment for third parties for at least two or three more years. Then there's the portability aspect of the Switch. A lot of people will be buying it just for that.
The problem with Wii U is that Nintendo wasn't quite sure how to market the thing. Third parties were as equally confused. Also, even though Wii U could play everything PS3 and Xbox 360 could play, the timing for the system's release was bad. Had it been released in 2011, that would have meant that third parties would have a little more until their games started running into roadblocks. Had Nintendo held off until 2013, they might have been able to put powerful hardware in the Wii U at a lower cost.
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