Because generally in ELEC EMER CONFIG the only time you're running on batteries is during the emergency generator coupling or on the ground below 80kts*.
Airbus say this because the possibility of both generators failing independently is miniscule. The problem is far more likely to lie with one or both AC buses, hence the ECAM actions of isolating them by opening the bus tie. If this doesn't restore normal power there's little point in starting the APU because it's just as unlikely to couple to the network.
On top of this (even if the APU ECB is powered on batteries only) attempting an APU start would almost certainly fail - next time you start the APU watch the ELEC page and notice the enormous load placed on the DC network. Would you really want to do that while these same batteries are keeping you airborne?
* Of course, depending on how antique your A320 is you could also be battery only during flight with the gear down. Same principles apply.
Last edited by Fursty Ferret; 23rd Aug 2012 at 07:05.