The Core Tier plays like CTF. But unlike an abstract Flag, the Core actually has Story value and brings the Game alive. In real life, a Flag has no value, and would therefore break cohesion with any campaign. But a Core brings realism to a campaign as a realistic target of interest.
Territory and Assault Tiers tell a story of pushing into the enemy strong hold. These can be effective in building the Story when they are played in Tier 1 and Tier 2. The Core Tier adds story of a heist from the enemy strong hold. Being difficult to find a reason to push into the enemy strong hold after stealing the Core, the Core Tier naturally fits in as the final Tier of a Round.
The Core Tier also naturally finds its place in Tier 3, where vehicular play is at its heaviest; where the Core Tier ends instantly with the crossing of the Core over the Objective boundary, with no contesting or disarming involved - giving the Game Play of pushing the Core over the finish line that climax of winning the Round.
Because the Core is a stolen game piece, the Defenders will defend the Core. The Defenders will do all they can to prevent the Core from being moved. Every inch that it is moved is one less inch of protection that sheer distance offers against the Tier's clock. Typically the only time you see the Defenders camping the Core Capture Zone is when the Invaders are in route with the Core and the Defenders do not know where the Core is at.
Since the Core Capture Zone is not the object of the Defenders' attention and since it sits external to their strong hold, it makes no sense to have two of them. There is no intent to divide the Defenders across the map. Since the Core Capture Zone sits in a neutral area of the map, it makes sense that there is only one such Objective for the Invaders to take the Core to.
Summary
The Core Tier adds realism to the Game Play as the finale to the Story in Tier 3.
The Defenders focus on defending the Core to prevent it from being moved rather than on protecting the Core Capture Zone (Objective).
The Core Capture Zone is in a neutral location and there is only one of them.