I am convinced the pilots saw the loss of “ norm brake “ on the ECAM ( indicates to pilots what’s not working due to the failure ) status page and thought they lost anti skid which would have greatly increased the landing distance. All they lost was normal braking but had alternate brakes with anti skid working. That’s what probably made them think they had to divert.
That said, no way I would divert with the landing gear either stuck down or not fully retracted given the much ( 180% ) higher fuel burn.
Even if the pilots did not notice the landing gear had not retracted ( by looking at gear light indicator ) , how they could not feel ( slow climb, much higher cruise power ) and hear ( wind resistance ) it was still down defies common sense.
It’s not hard to do quick math ( FOB….how much are we burning by looking at fuel flow …..how long to diversion airport based on ground speed showing on ND screen ) in a situation like that.
So, part of the problem was not understanding the aircraft well but also because they didn’t do basic calculations ( FMS flight management computer is unreliable with fuel predictions when the gear is stuck down and I bet they relied way too much on what the FMS was telling them in terms of how much fuel they would have upon arrival at the diversion airport. They probably first realized when the low fuel lights came on ).
