BA B744 vs B777?

I noticed on a recent trip to KLAX that BA were flying several flights (presumably from the UK?) using B744 equipment. Given the economies of flying the B777, and the fact that the routing is easily within the endurance of a B777, why does anyone suppose they are flying the older equipment? It could be the larger pax capacity of the B744, or the unavailability of the 777's being deployed elsewhere, but other than those reasons wouldn't the 744 be much more costly to run?Thanks,Bruce.