Because Micah’s crime was the same one Ezra had committed, only from closer range.
Micah had seen the forged signature and told himself a story that let him look away; Ezra had felt the wrongness in a decade of dead phone calls and told himself a story that let him stay in Texas.
Gideon was the one who acted.
But he had been able to act, year after year, because two other brothers had each found a comfortable reason not to look.
That is how these things last as long as they do — not because one person is monstrous, but because everyone around him is merely busy, or tired, or afraid of what the truth would ask of them.
Evil on that scale is rarely the work of one villain.