Since Grimaldo left the B team and went to Benfica, Spain has had Five different coaches such as Del Bosque, Lopetegui, Hierro, Lucho, and Moreno. Granted Del Bosque was at the end of his reign in 2016 but it still left plenty of time for his successors to look over the LB options they had and all of them never considered Grimaldo a serious option for the National team setup. Often going for the likes of Alba, Gaya, Alonso, and Monreal instead.
Back in 2015, there were rumors Lucho wanted to send Grimaldo on loan to Levante to see how he would fair but Grimaldo rejected which eventually led him to leave Midseason for Benfica. But all that shit was nearly 8 years ago, and I highly doubt Lucho even used that in consideration into not calling Grimaldo up.
Lucho loves versatility which likely played a part in Balde being called up as he's shown to be pretty adept at RB so far along with his natural position LB. A different profile to the likes of Alba.