Well, it's not just the swipe left I guess... even though I use that quite a lot. With the Google Launcher, Now isn't something that gets loaded up... Now IS the Launcher. The Google Launcher, Now, etc... that's all the Google app. The launcher app you download from the Play Store is just an extension. So even when you are launching Now via the On Tap gesture (or any other source mind you), it feels like it's part of the phone rather than just a slapped on application.
I dunno... it's just a speed and fluidity thing. I've tried Nova from time to time, to see where the development was going, and getting to all that info just wasn't as integrated. Now, if you don't use Now much, or just occasionally, that wouldn't be that much of a big deal. But it's one of the first things I check when I pick up my phone and go back to check those cards often.