All new watchOS apps will require using the watchOS 2 SDK 

Starting June 1, 2016, all new watchOS apps submitted to the App Store must be native apps built with the watchOS 2 SDK or later.

Apple’s Developer News states that all new watchOS apps need to be built with watchOS 2.

This shouldn’t come as a surprise, the original watchOS apps clearly were a stopgap until Apple could finish the watchOS 2 software. Since its launch there wasn’t a compelling reason to write any new watch apps in anything but the faster and more reliable watchOS 2.