People fall away from the Church for all sorts of reasons. Sometimes they rebelled when they were young: they rejected the Church because they were rejecting their parents and everything their parents made them do. Going to Mass was part of the package.
They might have made some life decision they knew the Church didn’t approve of — living with someone without marrying, for example.
In some cases, they might just have drifted away. They never made a conscious decision to leave, but after years of no involvement with the Church, they’ve stopped thinking of themselves as Catholic. Or they’re still “culturally” Catholic, but “not practicing.”
Most of these people have one thing in common: they think the Church doesn’t want them back. They think they’ve been away so long that the Church has rejected them. Or they think it would take so much work to get back into the Church’s good graces that they shrink from the prospect.