I was wondering that, the battery issues were after my time but I guess weight-wise it could be the case although perhaps in energy per unit volume they still win? As you can see in the pictures included in the Boeing fire-fighting document a few posts up, space is very, very tight, especially around the Aft EEbay battery, so more conventional battery technology was not going to fit without massive re-working.
That looks like a picture in an early airplane - ZA001 or 2 at a guess and it was tight even before the bullet-proof plate got added! 'my' bit of kit was the monster cardframe to the left of the battery and if you look closely you can probably still see bits of my skin from the happy months I spent pulling the darn thing in and out back in the day - and that was before the battery put on weight!