3.0 litre D4D Hilux question

I've just spent a few hours assisting the neighbours after the son put $30 worth of unleaded in a virtually empty tank on his D4D Hilux ute.
Neither of them are really mechanically minded, realised they shouldn't start it and rolled downhill and into the drive at home, then I got a phone call to ask what to do.
Advised them to drain the tank and then refill with fresh diesel.
Went over to see how they were going and we found that the plastic fuel tank doesn't have a drain bung, pretty clever on a 4wd!
Tried putting the jiggle syphone down the filler neck but there is a mesh to stop syphoning hoses.
So I went home and got a 12v fuel pump and a couple of lengths of fuel line and pulled the supply line off the fuel filter and started the pump, only got a few litres before it sucked air, seemed a bit weird!
So we pulled the filler neck off the tank (it was a traytop so relatively easy) and stuck the fuel line into the tank, sucked all the fuel out with the electric pump and then after reassembling the filler neck filled it with fresh diesel and pumped the fuel up with the priming pump on the top of the filter. It started first go and ran quite happily and continues to do so.
Can't explain the fact that you cannot suck the tank virtually empty by using the supply line to the fuel filter, why is it so? There was certainly about 30 odd litres still in the tank when it was sucking air.