You have received several good answers. I hope to add a little to the conversation.
-As mentioned, Protestantism can seem easy to understand. Converts to Catholicism often indicate they realized they previously had a truncated view of bible and tradition.
-One can also align one’s personal or political beliefs with one of the thousands of denominations. If you believe in remarriage without annulment, abortion, etc., you can find a denomination that accepts your view. This attracts the increasing influence of liberal politics and secularism on society. Catholicism, in its fullness of truth, does less to attract secularism.
- Protestantism seems to focus more on bible study, which is great. Catholics need more of that. At the same time, salvation is not completely an intellectual exercise. There is not a bible exam at the gates of heaven.
-Protestantism often places a higher premium on winning converts. While the Catholic Church has highlighted the New Evangelization, we tend to focus more on spreading the good news rather than winning converts. To me, this better reflects inherent credibility.
- I was catechized during the 1980s and 1990s. I have heard others say that, at some point, catechesis was watered down. It focused more on God’s love, the basic prayers, morality, etc. and less so on doctrine, dogma, etc. This is more or less how I remember it. Some suspect this may have been a reaction to the secularization of society, particularly in the 1960s and 1970s. In other words, they tried to react to a reduction of interest. One can argue whether or not it was the right course of action.
-** In a 1994 presentation aired on EWTN, Mother Angelica said that liberals had been trying to change the church. ** She said it had been happening for many years and that Catholics had been quiet for far too long. That was over 20 years ago. Additionally, we now know through Wiki leaks that progressives have been trying to infiltrate the Catholic Church to further their causes. I have little doubt this had some degree of impact.
To summarize, there are a variety of forces that have watered down Catholic catechesis and which ultimately lured people away from the Church. We have lost people to atheism, Protestantism, etc. However, this does not make the Catholic Church any less true. We are imperfect people making imperfect decisions, but Christ will be with our Church until the end. As you noted, there are people still converting to Catholicism and for good reason.
I hope to see a more balanced approach to catechesis for all Catholics, including RCIA, Sunday school, the media, and Sunday sermons. I hope they do not lose the message of love and morality, but increase knowledge of history and doctrine. I have read the seminaries are reverting to a more traditional approach, and I think that is a good thing.