Because the plot was badly-written.
Hiruzen didn't move a finger to befriend the Uchihas even though he wanted to. He was too afraid to move without the agreement of the council. Itachi was an idiot as well for not exposing Danzo and talking to Hiruzen and maybe even to Fugaku to set up a meeting so they can actually discuss intentions.
The only ones that wanted the Uchihas to stay caged were Danzo and those two fossils, Hiruzen was just too passive to do anything about the Uchiha clan.
He could have easily talked to Fugaku and told him "man, I want the Uchihas to feel welcome and at home within the leaf village" yet he didn't. Fugaku was also thinking about peace but he also didn't go talk to Hiruzen.
Overall, it was just bad writing.
Also, the coup was coming close because nobody was giving the Uchiha clan anything, they were kept at the border of the village, they could have negotiated things so easily but they didn't.