They announced the X1 because they had no customer lined up at the time of announcement. The timeline for when the X1 was revealed matches closely to when Iwata started talking about the NX, which was some months later, so the deal was inked very early but not before the announcement.
Switch Pro SoC, if it's a new one and not a Mariko shrink running at even higher clocks, likely will not be announced before hand in all likelihood as they have already secured a customer and it is in the customer's interest to not have their big reveal spoiled.
A rough timeline on the consumer side of things. Note: "Erista" is the code name for Tegra X1.
March
2014: - Erista added to Tegra roadmap, will be Maxwell-based. This will be the successor to Logan, the Tegra K1 (Kepler based).
January
2015:- Tegra X1 "mobile superchip" is announced
March
2015: - Nvidia Shield TV micro-console announced, will use Tegra X1
March
2015: - Iwata mentions the Nintendo NX
May
2015: - Nvidia Shield TV is released
October
2016: - Nintendo reveals the Switch
March
2017: - Nintendo Switch is released
Also note (most of you know this but for others that don't) Nvidia never officially had a product called "Tegra X2"
Their Pascal-based SoC for automotive/AI stuff was codename Parker.