https://screenrant.com/young-sheldon-characters-big-bang-theory-ignored-reasons/

Despite being set before its parent series, some Young Sheldon characters not only didn't appear in The Big Bang Theory, they weren't even mentioned.

Sheldon’s grandfather is said to have been the person who inspired and encouraged Sheldon to be interested in the sciences, and after he passed away, it seemed like Sheldon was destined to be the only person in his family interested in science. Dr Sturgis is therefore the perfect figure, almost like a substitute grandfather, to continue challenging and nurturing Sheldon in his interests. This is even more poignant because Sheldon does not really have any other family member who is interested in Physics like he is, and the other male role models in his life (George Sr. and Georgie) are more football/auto mechanic fanatics. Dr Sturgis also does not judge that a nine-year-old is smart enough to do college-level classes, but just accepts Sheldon into his class. His relationship with Connie only brings him closer to Sheldon. Dr Sturgis is also somewhat like Sheldon himself, where he finds the science in everything and says the weirdest things, like when he says he chose to have vanilla ice cream only for dessert because it saved him two and a half days of decision-making by having made the decision already. It can come across as funny, and it also makes him an endearing character.