
I don't know the answer to that. And there is no answer given in the book. So I asked around to some people that I respect, and here are some of the answers I got:
Anon 1: Nephi saw that there was already plenty of contention, and that it only led to disaster and getting lost, so he just let it go. He knew that it was supremely important that he get, and keep, control of his emotions.
Anon 2: Just cuz the scriptures don't mention it specifically doesn't mean that it didn't happen. I mean, he wrote that probably months or years after it happened. History is kind to us when we write it ourselves. :D
Anon 3: Nephi is superhuman; he was "born that way." The Lord knew that he'd need someone patient who wouldn't complain in order to lead this whole troop across the water. Nephi just had a kinda "natural faith" that always saw the eternal perspective.
Anon 4: Nephi knew that they were only suffering from the effects of his bad, and somewhat rancid, cooking. Mad cow disease - or maybe Mad Camel Disease.

Anon 5: Nephi really struggled with being ticked off and wanted to toss them off the boat. He probably had the muscles to do it as he was strong and had just built a boat for gosh sakes! But he prayed for strength beyond his capabilities to deal with a bunch of idiots who just weren't going to change, angels, floods or otherwise. He just figured, "We're all in the same boat." Literally.
It seems that the definitions fall into one of two groups: Some people are just born with natural patience - its in their DNA, OR patience is a virtue that you can, and should, develop until you can just about drown with your family without complaining.
Maybe it's just that I tend to be an angry person - or rather, I have a more generous definition of what is worth getting mad about; but I just feel at a loss as to what to tell these kids about anger, and complaining. Maybe the art of "sucking it up in silence" is just becoming a lost art? I dunno. But while you're thinking about it, please pick up your shoes and put your laundry away, I've already asked you twice and I'm gettin' a little fed up with the mess!!!