if american airlines put a plane in st martin that was due for a long haul and could not carry sufficient fuel because of weight ( which had to be because of added frieght loaded) they are way behind the curve on planning and i would be tremendously concerned as to how in the world they could allow that to happen. it just is not what a professional air carrier does!!
SXM PHL is not "long haul". The A320 is perfect for that mission.
Regarding fuel: it might be cheaper in SJU, but that's not the reason for the unscheduled stop (an expensive operation for any airline).
Performance calculations are made by pilots before every take-off. The performances are highly depending on weather conditions.
1. Wind direction
2. Outside Temperature
3. Atmospheric Pressure
4. Wet or dry runway
Planning departments use "average weather conditions" for their calculations and cannot cover every scenario.
I don't know what happened on that day. But all I can say is that in the last days there were strong Northerly winds over the Atlantic. This means headwinds when flying Northbound. The headwinds increase flight time, which in turn means more fuel needed for the flight. It might explain why, with the specifics of SXM (the short runway and the obstacle after departure), the 320 could not carry all the necessary fuel for the flight.
Also, bear in mind that pilots decide on how much fuel to take depending on the weather conditions at destination. If the weather is crap (which might not have been necessarily the case on this one), more fuel is added to the minimum flight plan fuel, so as to cover for any unforeseen circumstances (holdings, diversions, etc...).
It's not always as "black and white" as it might appear, although I understand the frustration that delays can sometimes provoke.