Because it will cost an absolute bomb. It's been looked at several times - and is recognised as the most likely option to meet LO's growth in the long-term. However there's a lot of stuff on the trackbed that'd need to be moved first and it'd also need a bit of jiggling with the track layout, which is best left until the migration to the ROC is complete (is that area still controlled by Wimbledon?).
IIRC the current platforms 1/2 can handle 4tph easily each, with 5tph being fairly safe but 6tph is pushing everything to the minimum margins and would likely be problematic for performance.
EDIT: Note the different structures for the Platform 0 trackbed and the main station arches:
https://goo.gl/maps/ktFNnvnd9pEph8Pg9. Certainly possible to refurb it, but not cheap in that location.