Diversion choice
Could it be (and I am certainly no expert) That American may have better access to engineers and ground handling back at 'base' (in this case Heathrow) and better options to transfer passengers onto alternate flights from LHR for those who wanted to? If the situation on board is considered stable enough and the cabin is still secure then why divert to an unknown airport when you can just go back to where you started (fuel permitting)?
Granted we don't know what went on and all of our comments are purely speculative on here but if the affected passengers were not worsening, if I was in that situation I would want to return to LHR.