Newman said "to be deep in history is to cease to be Protestant." Why don't more Protestant historians become Catholic?
The short answer
Newman's maxim is a general observation, not a rule. Belief in Catholicism is ultimately a gift of faith that must be accepted and acted on. Some may not have received it; others may not have accepted it. We commend them to God's mercy, trusting that those who follow him according to their light may be saved.
An observation, not a rule
Newman did not mean that every Protestant versed in Church history must become Catholic. He meant that history argues against Protestantism and that many who study it deeply come to see the Catholic Church as the true Church.
A gift of faith
Belief in Catholicism is finally a gift of faith from God, which must be accepted and acted upon. For reasons of Providence some historians may not have received it, and some who did may not have accepted or acted on it.
God's mercy
We commend such people to God's mercy, trusting that if they follow him as best they can by the light of truth they have, salvation may be possible for them (see Lumen Gentium 16).