Because 85% of a bill is set by regulatory, tax, and supply costs, which are pretty much the same for all big six suppliers (The smaller companies get tax benefits such as not having to pay the green tax)
10% is running costs, and 5% is profit.
The only way to avoid having to all change prices at the same time would be to significantly lower profits, or cut running costs (e.g. redundancies, which will affect service levels and innovation)
When the core product is being sourced by everyone from the same place at the same cost, of course prices are going to be similar across the board.
So this year's rise is nothing to do with the Labour announcement then. Further more the amount of increase has not jumped because of the announcement. Either BG don't think Labour will get in or they are saving the price hike to offset it for 2014 - which will prove the above to be nonsense.