Because its so damn cool.
but my guess from wikipedia:
Main spar location[edit]
The aft location of the main wing spar would lead to a more efficient interior arrangement with more usable space.
Inward spanwise flow[edit]
Spanwise airflow over a forward-swept wing is the reverse of flow over a conventional swept wing.
Air flowing over any swept wing tends to move spanwise towards the aftmost end of the wing. On a rearward-swept wing this is outwards towards the tip, while on a forward-swept wing it is inwards towards the root. As a result, the dangerous tip stall condition of a rearward-swept design becomes a safer and more controllable root stall on a forward-swept design. This allows full aileron control despite loss of lift, and also means that drag-inducing leading edge slots or other devices are not required. At transonic speeds, shockwaves build up first at the root rather than the tip, again helping ensure effective aileron control.
With the air flowing inwards, wingtip vortices and the accompanying drag are reduced. Instead, the fuselage acts as a very large wing fence and, since wings are generally larger at the root, this raises the maximum lift coefficient allowing a smaller wing. As a result, maneuverability is improved, especially at high angles of attack.
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