...Infinity Stone could suddenly be able to give two people such specific powers. And there was no precedence. It's not like the scepter had a dial on it that said Choose A Power. =P
Some excerpts:
"according to a memo written by Strucker himself, the Maximoffs were chosen as test subjects due to the fact that they had the "appropriate genetic markers [that] make them conducive for human trial volunteers." In a subsequent post-mortem, Dr. List (Henry Goodman) ponders why Wanda and Pietro proved to be such a success where "other subjects failed and perished."
"the Marvel Studios Visual Dictionary from 2018 states: Whether it altered her or merely unlocked something latent inside Wanda, the Infinity Stone on Loki's scepter bestowed incredible powers of the mind." "
SyFy