The last few years has certainly seen a major decline with BE being the only operator showing any major increase, and they're mostly only covering the routes once operated by others.
Gone are the days where BACX/BRAL/BD etc would send 5 or 6 a/c each from different destinations into JER which would then rotate through the island all day.
Its down to several reasons; most recently the economy and the rise of extremely cheap intra-european travel. Moreover the die-hard pax that used to visit Jersey year in year out (read 'the oldies') are 'dying out' so to speak. Although this sounds sinister its true, and there are less people to fill the charters. Although Jersey is trying to market itself as a destination for the younger couple, I don't think it'll ever truly succeed in this market, and will inevitably see a further decline in the future. This is especially true of the UK market.
Gone are the days of dispatching Aer Lingus 737s or British Midland F100s from the Freight ramp sadly!