Maybe Nico was offered a much higher salary which Barcelona can't afford to pay him currently or could be anything, no one knows his reasons. If not for form Rashford is a much better option than Nico, he scores a lot of goals and also a very skillful player. I also have same feeling that Rashford will do well at Barcelona because he plays a much similar style to them and will enjoy all the spaces they create.
In my opinion, I think Athletic Bilbao never wanted Nico Williams to join Barcelona in the first place; that's why they made it difficult for Barcelona to sign Nico Williams, and when Athletic Bilbao found out that Barcelona couldn't pay Nico Williams's transfer fee at once, they quickly rushed to give Nico Williams a lucrative 10-year deal with a new release clause just for Barcelona not to sign Nico Williams.