It may be presumptuous to say the aircraft don't have the range. Full fuel isn't automatically carried on a long journey. Extra fuel tankered for a trip is expensive, and gets needlessly burned. Typically 4% of the fuel tankered gets burned, just to carry that extra weight, though this can be substantially more, depending on just how heavy the aircraft is.
Winds aloft can be strong. I had 200 knot winds recently over the Pacific. North Atlantic winds are not usually that strong, but going westbound certainly slows a flight. For flights that are conducted under re-release rules, the key issue is the amount of reserve fuel necessary for a long Class II navigation segment (the oceanic part). On a re-release flight, the total fuel reserves are reduced, provided that by a certain point on the trip, the company is contacted for an update and recalculation of the flight performance data. If the crew and company jointly determine that adequate fuel remains to make the remainder of the journey, then the flight continues.
If, however, a determination is made that fuel burn is too high, a diversion will be necessary. Evidently this flight diverted for fuel.
It's important to note that the flight wont' be operated at any time unless enough fuel reserves exist to be safe, but the re-release function on certain flights is there to allow a flight with reduced fuel for efficiency. Clearly a stop isn't efficient, but where field conditions are different than forecast and other events may conspire to increase fuel burn, a stop may be necessary.