I believe it is just an operator requirement, the obvious reason being that if the weather is bad in Paris LBG for instance, its not likely to be much better in Paris CDG etc.
No other reason I could think of, as it would of course be favourable for the pax to go near to destination.
The only other consideration is that the alternate fuel is the minimum required by Jar ops (type specific) or sufficient to reach an alternate if further away.
