[Ali Imran]

People who read the scriptures were Christians 
So he had to ask the Christians 
It says ask the people of the book the authors
It didn’t say ask his own people 
His own people were moslems 
Moslems don’t read the Bible they only hamtum it like u

Not the Christians but the Israelites. 

And it had to be the Israelites who were already with the Prophet صلي الله عليه وسلم, like Abdullah bin Salam. The other Israelites were hostile towards the prophet صلي الله عليه وسلم.