The key factor in Nico's decision was a WhatsApp message sent by Barcelona. The message reportedly suggested that Nico should drop his agent, Felix Tainta, and instead sign with a different agent, such as Jorge Mendes or Pini Zahavi, who are close to the club.
The suggestion was met with outrage from the Williams family, who are close to Tainta and also take care of Nico's older brother, Inaki. The family's strong relationship with Tainta was likely a significant factor in Nico's decision to stay with Athletic.