This is a good question, and you may find the answer by watching Marcus Grodi's program - The Journey Home - broadcast live, Monday evenings at 8pm, on EWTN. In fact, since this is a live telecast, you may call in the question or email it to journeyhome@ewtn.com. Either way, you have a good shot at receiving a response.
From years of watching and listening to these conversion stories, a certain 'pattern' has emerged. Many are called but some, for personal reasons, reject it. It's a difficult and oftentimes painful road down which to travel. Conversion can result in the loss of family, friends and stipends. Take for examle the case of Alex Jones, a former Pentecostal minister, whose story you can read here.
There are also female Protestant ministers who recognize the Catholic Church but are reluctant to give up their ministries, recognizing that they can no longer serve in that capacity.
No, I cannot accept "many are called, and few accept"...there are too many problems in the stories I hear on that show.