I am going to use Oneness Pentecostal as an example, as I know a former Catholic who became Oneness Pentecostal. His reasons: Oneness theology, though not perfect, made more sense to him than Trinity theology. Catholic rites are mostly calm, whereas Pentecostal rites are more exciting. They encourage speaking in Tongues and the spiritual boost this gives them.
This is why I am always noting that God meets us where we are and draws us ever closer from there. I came across The Book of Mormon before I was even aware of the LDS Church. While looking through it, I did feel a spiritual connection of the Spirit saying, "This is not for you." I always emphasize that the Spirit said, "Not for you," and did NOT say, "Not for anyone."
I have great faith in the power of the Holy Spirit and a great fan of staying out of his way. I have encouraged both goddaughter to return to her Jewish faith, and my daughter to study the Jewish faith. I believe everyone should seek and find God. I found Him in the Catholic faith, but keep in mind people like Abraham, Noah, and Jacob found Him in the Jewish faith. My Oneness Pentecostal friend drew more closely to God in that faith. None of the above ever once felt they had to tell me how wrong I was in my own faith.