Why are so many Mormon actors inactive - or, conversely, why are so few Mormon actors active?
Rick Schroeder shared in an interview once that it is very difficult to be successful in Hollywood if you don't socialize like the others in the industry do.
I think that's the major reason - and, of course, the need to be willing to take roles that don't align with basic Mormon standards. People generally don't like to be around other people if they think those other people don't approve of them - if they think those other people are condescending. Having different standards of socializing doesn't mean condescension exists, but impressions are reality for many people.
Also, it's hard to work regularly at a really high level in Hollywood if casting directors know you won't take a role that has nudity or a sex scene, for example (or where the character drinks or smokes - or swears, in some cases). Generally, you need to have street cred and be highly-desired over others to get away with having personal restrictions. I know not all Mormon actors have those personal restrictions, but I'm sure it's a general stereotype in the minds of casting directors by now. It's also impossible (or, at least, extremely hard) to make a living in film and stage, especially, if you want to attend church regularly on Sunday. It's no harder than being a professional athlete during the season, but there isn't a scheduled "off season" in film and theatre - unless you are a top-level actor who can pick and choose and create your own personalized work schedule.
My oldest son was studying to be an English and Theatre teacher, and he was asked once by the guest artist (a professional actor) who was working with them in their production why he wasn't trying to become a professional movie and/or play writer. He was flattered by the question, but his answer basically was two-fold: 1) he wanted to be a teacher / professor; 2) he didn't want to give up his religious life (the activities of regular attendance) and family life (the ability to spend quantity and quality time with a wife and children).
Also, when it comes to Hollywood, Proposition 8 didn't help, I'm sure.
In summary, to get steady work, it's almost necessary to make it clear that you aren't an active, practicing, steroetypical Mormon. Katherine Heigl, for example, has said that she might return to church activity once she stops acting - but that she'll have to give up the alcohol and language in order to do so.
I wish she felt she could return to activity before giving up those things, and, to me, that's an even more important issue. The perception that she can't attend our meetings and worship with us while not following the Word of Wisdom and being a stereotypical "good Mormon" is one reason for inactivity that we can address - one constraint that we could eliminate, if we all understood and truly embraced charity for what it really is.