why the hell a plane would leave an airport without enough fuel to make it to it's destination?

My kids' grandfather died Wednesday evening, in Nebraska....and my son, daughter and granddaughter - she's 7 months old and still nursing (SIL and grandson stayed here) left this morning on a 6:20am Delta flight.
They were scheduled to fly from here to Minneapolis, change planes and fly into Omaha, to be met and drive another 2.5 hours to Grand Island.
Their plane diverted to Kansas City ....BECAUSE THEY NEEDED TO REFUEL!!!!
AREN'T THEY SUPPOSED TO TAKE OFF WITH ENOUGH TO GET WHERE THEY ARE GOING?!?!?!?!!
Because they were diverted to KC...they missed the connection out of Minneapolis to Omaha.
When I talked to my son about an hour ago... Delta said they couldn't get them out of MN until 7:30 tonight!!
He'd already had the Delta person hang up on him and was waiting on hold to talk to someone else, and they answered while we were talking so he hung up.
Their phones are going to straight to VM ....so hopefully they got on a flight and their phones aren't just dead...

At least they are together....my daughter would be flipping out if she was alone, especially with the baby...
Good thing Delta wasn't flying over the ocean...