Data and Lore were the exception to the rule because they were so intricate. It's not like anyone could whip out something as perfectly articulated as the two of them. They were more just the hope that Soong's Positronic brain would work. He failed epically with B4 and a few others, apparently. Lore was the first success, but he was deeply flawed in the sense that he felt too much and no one taught him how to control his feelings. So, when Soong tried again with Data; he had the formula down to a T.