Partly because the limit of exploration is a blocked descending rift less than 15 m above the level of the resurgence at Newhouses Rising (which is impenetrable) and there is no draught. The resurgence is about a kilometre away.
Some time ago, before this final area was discovered, I observed a flood pulse emerge from Newhouses Rising less than 2 hours after rain started on the fell. However dye takes quite a long time to get through. This rapid transmission (together with the exceptionally low gradient and long transmission times of dye) suggest that much of the cave system in between Sell Gill and Newhouses will be flooded.
But as they say, caves is where you find 'em (quote P.B.Smith, on many occasions over the years).