He wanted Phosius to settle down, Phosius wanted independence and used filioque as blackmail. Pope John VIII didn't see any problem in accepting some conditions from the east in order to avoid schism, but Phosius and the Church of Constantinople kept on their fight against the supremacy of Rome.
in 870 the Byzantine Empire was very strong and Phosius used the power of the empire to negotiate with he pope:
But for the years of the council of Florence the bizantine Empire was like this:
In the years of Pope John VIII Constantinople almost ruled over Rome. But in the Years of the council of Florence, Otomans were almost ruling over Constantinople. With Focious Greeks were arrogant, but with Ottomans they signed Filioque.
It was politics, Besides, for the years of Focio, The first council of Toledo had already passed almost 500 years.
By the way the Church of Constantinople rules over 5000 christians in Turkey, and even though that is called a Patriarchate. According to Wikipedia there are 15,000 Catholics in Istambul. almos 3 times the number of Orthodox.
All around the world Constantinople has 1'500,000 members. Even the church of my city has far more members, in Guadalajara. Something is just very wrong.
Even the Spanish church in the worst years of Muslim oppression was able to reject Muslims. Even with filioque, but since Phosius Constantinople was constantly falling.