I'd blame Catholic guilt for some of it. It's one of the hardest things to shake when becoming a former Catholic, that feeling that everything's really your fault, somehow.
Former Catholics seem to only rarely convert to another religions. More often, they become agnostic or atheist. It's like Catholic catechism either makes you one of the faithful, or burns the desire for organized religion out of your system. There's sort of a middle ground, that of being a lapsed Catholic, but that's sort of like Limbo*. Some lapsed Catholics will attend mass on High Holy Days and other occasional events, often out of a sense of duty to others (usually related to the guilt of upsetting someone), and others will cruise along, until asking for a priest on their death bed ("Bless me Father, for I have sinned. It's been 34 years since my last Confession....). I said that I was a lapsed Catholic for a long time, until I finally realized that I had essentially become an agnostic (or a Deist on sunnier days).
*Except that Limbo's been removed from Church** teachings after 800 years, but you get the idea.
**See, there's that Catholic dogma again. There's only one true Church, and we're still a little upset over that upstart, Martin Luther.