Why?
The Gospel of John has seven signs.
John prefers to call them signs since all events are exterior and interior by nature. This accounts for the different reactions and responses people can, and do, have to them.
The healing of the blind man from birth is earthy, tangible and dramatic:
As Jesus went along, he saw a man who had been blind from birth. Jesus spat on the ground, made a paste with spittle, put this over the eyes of the blind man, and said to him, ‘Go and wash in the Pool of Siloam (a name which means ‘sent’) (John 9).
The blind man responds well and believes in Jesus. The Pharisees react badly and do not believe.
Why?
St. Thomas Aquinas (1225-1274) taught that two things are needed to believe.
First, an exterior event such as a miracle or some revelation through teaching or preaching that arouses interest.
Second, an interior grace is required.
God has to act in the soul, preparing and tilling it for belief:
We must assert another internal cause, which is from God moving us inwardly by grace (ST, 2-2, q.6, a.1).
We best try and be sensitive to this inward grace.
Amen.