It's simple. The Apple Watch is nowhere near as popular as a new iPhone. Less people will be pre-ordering it.
Use the Apple Store app, it's much quicker.
It's safe to assume that they've made far less Apple watches than they did iPhones so you could realistically expect it to be just as hard to get one at launch.
Their pretty much the same speed. Even though the app doesn't have express checkout, all you need to do to finish the purchase (provided you have your CC saved to your account already) is put in the 3-5 digital security code from the back of your CC. That'll confirm and complete the purchase.
from experience app is a more stable experience and quicker. Also in the past it has accepted orders before the web.