Because Proactive can search within apps, something that won't be seen until apps are allowed to update for iOS 9.
Spotlight Search can't do this.
At least, that makes the most sense. We won't know until iOS 9 comes out if i'm right.
Either way, I still don't see why this is such a big issue? They're both there, and it's not hurting you, nothing is being lost, and personally I've been using both. I swipe left when I want to see the suggestions (and I sometimes search from there) but if I'm quickly looking for a contact or an app I pull down, since the keyboard launches with it. There's no loss or gain from having both, so I don't get the concern.