De Jong is a weird player who is neither Pep's 6 nor 8. Gained his reputation at Ajax playing as a 'libero' shifting between defence and midfield. He's basically a 'Pirlo' with a dribbling ability, a deep lying playmaker whose main strength is connecting defence and attack.
As an 8, he hasn't set the world alight at Barca, lacks playmaking instincts in the final 3rd and is currently in the shade of his teammate Pedri. As a lone 6, he isn't positionally disciplined enough, so old and barely running Busi is preferred.
Initially Pep wanted De Jong as a lone pivot material. But now, having fully integrated Rodrigo, we simply don't need such an expensive understudy to him. And don't forget he had chosen Bartomeu's Barcelona over Pep's City project and was happily fucking about in Spain while Rodri worked hard to improve under Pep's guidance. Why would you want such a player in the team?
Besides, you have to take squad planning into account.
We have enough players for two '8' spots - KdB, Grealish, Bernardo, Foden, Gundo. Add Bellingham as a rumoured transfer target next year. What we need is Rodri's capable backup. Reliable, but knowing his place in the hierarchy. Phillips arguably suits it better than glamorous FDJ.