** WARNING: SPOILERS for Star Trek: Picard Ahead**
The reason given in-universe was ALL positronic-type developments had to cease and any androids/robots, ie Synths, disassembled after the "attack on Mars", by the order of Federation council.
Since it was initially thought to be a programming error in the synths, the Federation Council placed a ban on ...
Soong did not actively keep his work a secret ... at first
In TNG: The Schizoid Man, Dr. Graves is able to recognize Data as Soong's work, and claims that he taught Soong:
GRAVES: No insult intended, Mister Klingon. You see, Doctor, there's nothing wrong with my hearing. My, my, my, my.
DATA: Lieutenant Commander Data
GRAVES: Shhh! Absolutely no ...
It's likely that he did, but he didn't consider B4 a "brother"
Actually, Data and Lore have three older "brothers", B4 and two unnamed Androids. These androids are referred to as prototypes by Soong's wife and co-worker:
JULIANA: How do you know the same thing won't happen? Creating a stable positronic matrix is very tricky. Your father lost several ...