What? That's absurd. Timed exclusive patches now? Please tell me you're joking otherwise i'm going to be sick. Timed exclusive DLC is one thing and i'm ok with that. Holding back added features from platforms is disgusting at best.
Sadly I'm not joking. The reason is most likely because (sorry, I haven't read 1.6 patch notes) 1.6 also includes compatability for Dawnguard. That's the only plausible reason, as that's the only reason I can see why they would wait to release it.
And in response to your comment below, no, you don't need DG for mounted combat.