One other possible reason: maybe IAG don’t want another group buying EI and possibly asset stripping it in much the same way that LH did to BD (before 2009, BD had 14% of LHR slots, in 2012, 8%).
I'm a fan of this logic. IAG buying EI is a defensive move in my opinion. They keep the UK Regional/Irish pax revenue in the group AND prevent anyone else making inroads into the UK market or increase their presence at LHR.