You forgot two very important things, Malte279:
1) when Bron left, Mama Longneck was pregnant - Bron says "I knew we were going to HAVE a child" meaning that she had not given birth yet. A pregnant Longneck would be a colossal risk to travel with, especially as it was just Bron alone; if she got hurt or went into labour on the journey, she'd be a big risk - the noise would attract predators from miles around. Or, what if from lack of food on the journey, she lost the baby? Or what if Bron got injured and couldn't protect her and she died when attacked? Yes, where she was would have been dangerous, but there were lots of Longnecks around to protect them until they got separated by the Earthshakes.
2) Bron doesn't KNOW Littlefoot is alive; he stumbles across him by sheer accident. He mentions that he met someone who was able to tell him what had happened to Littlefoot's mother, but could not say whether Littlefoot was alive. For all Bron knew, his child had died some time after being born. Bron knows the child was born, because he says that he searched for Littlefoot, and Bron would probably know when his child was conceived and due to be born, but he doesn't know if the child lived/escaped a Sharptooth, etc.
Over time, he gets a herd and doesn't find Littlefoot. As he has a herd and no definitive status of his child's existence, he goes on with the herd; safety in numbers, no-one else to look out for, etc.
Bron has to work on the basis of "My child was born, but his mother died. An infant doesn't have any chance against a sharptooth, so he must be dead".
THAT is why Bron doesn't come to the Great Valley before. It's a complete accident that Bron even finds Littlefoot at all!
As well as this, Grandma and Grandpa and Littlefoot's mother and Littlefoot himself are referred to as "Longnecks"; Bron is NOT. Maybe he's a different species that wouldn't be classified as a Longneck? Maybe a Camarasaurus or something? And that is why he didn't have the sleep story - he's NOT a proper Longneck!
Or, he's got his herd to look out for and didn't find Littlefoot. Or Grandma and Grandpa Longneck thought Bron was dead -- Grandpa is shocked to see him in 10, so it's plausible.