On a recent Journey Home with Marcus Grodi program (on EWTN), a guest made the following observation (which I paraphrase):
1) The Protestants converting to Catholicism tend to be people who understand theology, Church history, etc. quite well, and who are seeking to take their Christian faith to a deeper level.
2) Catholics that leave the Catholic Church tend too be poorly catechized and never really understood well the deeper aspects (mysteries) of the Catholic faith.
That sounds right to me, at least from what I have observed here.