I'm sure this was more a airspace availability issue than a fuel saving issue. Putting a packed 747 in heavy tubulence can create quite a risk for passanger injury, something no pilot desires. Such trans-oceanic routes are fairly narrow and with so many flights it's far more likely there simply wasn't airspace available. Also, (although it might not apply in this case due to your proximity to land), changing clearance in the middle of an trans-oceanic crossing can take a little while (it's not as simple as getting on the radio and requesting a new altitude).

-FlyerBeek