K.W.
It shouldn't be presumed that if someone decided to stop homeschooling, it was because "it wasn't working." Educational needs change over time. It's okay to move from homeschooling to other kinds of schooling (and back) as needed to meet the needs of your children.
The number one reason I hear from parents for moving from homeschooling to public schooling is child request. If a kid expresses a coherent interest in going into public school, most parents will let them try. Usually, that goes well. Sometimes the child does one year and then asks to homeschool again.
I also hear from a lot of introverted parents who choose to enroll their extroverted children in public school. A parent who prefers to have playdates 4-5 times a week may find it difficult to meet the social needs of a child who prefers to have playdates 4-5 times a day. This is also an issue when there is a significant difference between kids. When a parent has one child with significant social anxiety and one child who is an extreme extrovert, the extrovert often goes to public school.
Sometimes parents/kids are seeking specific resources: sports, music, drama, science lab, etc. This is especially an issue in high school.
Sometimes parents discover that their teaching style is simply a terrible match for their child's learning style. For example, a highly auditory parent who teaches by speaking may have trouble providing good education (or communication) with a completely visual child who must see everything.
Money and schedules. Disagreement between parents.
Those are all the issues coming to mind right now.