Okay lemme see. Here are these people, raised Catholic then drifted away. Now why did they drift away? I think a lot of times (not ALWAYS but I bet in many cases) it's because of becoming a teenager and all the experiementation and rebellion that goes with it. Then on to college and young adulthood and giving in to the temptations of illicit sex, divorce and other sins that result in a even greater breech with God and the Church and even more neglect of one's spiritual life. Happens quite a bit to people in their 20's and 30's.
Then one day these people are older and wiser and decide they need to settle down and tend to their souls. A good thing! Along comes an Evangelical and off they go with him to an Evangelical Church just to see what it is like. Well it's great because Hey you don't have to go to Confession and confess 20 years worth of sins and the fact that they you are on your third marriage. (Things the Catholic Church certainly did not make them do!)
And so they find God. (Again a good thing.) But they start to look back for reasons why they lost him. Not wanting to blame themselves for their drifting away from Jesus in the first place, they decide they never knew him at all and this was not THEIR fault, it was the fault of the Catholic Church.
My speculation, and it is just that, is that hardly anyone leaves the Catholic Church while in the state of grace, that is, while living a virtuous and godly life as fully participating, practicing Catholic. They drift out because of sin and forget that is why they left, then blame the Church for not bringing them close to God. But the fact is they closed THEMSELVES to God or they would not have left because God and Jesus are right there in the Catholic Church!