Games announced today were probably planned a year before Switch even launched. 3rd parties hedging their bets for another Wii U were not going to put resources into a Switch SKU. Now that it has proven a success, it takes time for 3rd parties to plan those resource allocations.
The rest of the reasons in no particular order.
Lower power/different architecture
Game cards (price and capacity)
Relationship with 3rd parties is weaker (though getting better).
Switch will get better 3rd party support in time but it's too early to expect a lot of it now.
Massimus - "Trump already has democrat support."