Considering Missy’s prominence and popularity on Young Sheldon, it’s curious why The Big Bang Theory didn’t take advantage of this and feature her more as they did with Mary.
However, there are a couple of answers to this query.
Narratively, it didn’t seem like Sheldon and Missy were all that close when she first appeared, so it made sense that they had limited contact, whereas Mary usually sought and coddled Sheldon. Given their strong bond, it’s understandable that both regularly kept in touch.
It’s also worth noting that after visiting Pasadena for the first time, Missy eventually married and had kids. This kept her busy for years, making it difficult for her to just travel to California to hang out with Sheldon.
Behind the scenes, CBS didn’t know how popular Missy would grow to be in Young Sheldon, meaning the network didn’t make a conscious effort to bring her back more frequently in the long-running sitcom.
It would have been better for The Big Bang Theory if it featured Missy more frequently. Seeing the socially-inept genius’ different dynamic with his twin would add a layer of complexity to his character as someone could either back up or debunk Sheldon’s childhood tales, especially regarding patriarch George (Lance Barber).
Beyond The Big Bang Theory, it would have also greatly benefited Young Sheldon.
Instead of just Sheldon, the prequel series could also put more focus on his sister as it highlights just how much she has changed (or stayed the same) through the years.
As CBS continues to make Young Sheldon more of a family dramedy than a straight-up sitcom like The Big Bang Theory, audiences can reasonably expect more focus on Missy moving forward.