Why don't Mormons talk more about some of their more controversial beliefs? Why aren't Mormons more up-front about some of the more controversial doctrines of the LDS Church? The answer is simple: they are taught to be circumspect about many Mormon teachings and to avoid getting into too much detail with outsiders — especially prospective converts.
This attitude towards what's said about Mormon teachings has even apparently led to some Mormons thinking that the sanitized version of Mormonism — the version for outsiders which is vague or even silent when it comes to controversial teachings — is the "true" Mormonism. Thus some Mormons don't realize that they are expected to believe that God was once a human man, that human men today can become gods, that God is married and has sex, etc.
Embarrassing Mormon Doctrines?
On the one hand it may seem natural and obvious that a group would avoid talking too much about its more controversial doctrines and ideas.
However, we're dealing here about a religion where those doctrines are supposedly handed down by God not an embarrassing uncle who talks about aliens when he drinks too much at family gatherings. Avoiding talking about an idea or belief usually implies that a person is embarrassed by it or even ashamed of it; at the very least, it implies that they recognize there is something "off" about it.
If, however, a person sincerely believes that a doctrine has been handed down by God for everyone to adopt, shouldn't they be happy to spread it and get more people to believe it? Even if they don't make a point of telling everyone they meet about their beliefs, why avoid talking about these beliefs when asked directly about them?
People who believe orthodox, traditional Christian doctrines which are at odds with modern science and logic — like creation of the universe in seven days, Jesus' resurrection, or the Trinity — don't evince any embarrassment and rarely, if ever, avoid talking about those doctrines (at least when the subject comes up, and some go out of their way to share this information).
Mormon Missionary Training
Avoiding discussion about controversial Mormon doctrines isn't something that's done implicitly or accidentally. According to former Mormons, it's something that's taught quite explicitly during Mormon Missionary Training.
All Mormons are expected to do missionary work to convert more people around the world and get them to join the LDS Church. They are expected to do so, however, without going into too much detail about Mormonism's most controversial teachings.
Prospective converts have often heard of those beliefs, however, which means that Mormons are also taught what to say if asked about them — what information to provide, what to avoid, and how to answer hard questions without being any more specific than absolutely necessary.
Milk Before Meat
The phrase "milk before meat" is used in Mormons' missionary training to describe (and, arguably, rationalize, justify, and excuse) the practice of telling prospective converts (and outsiders generally) only the simplest, easiest, least controversial teachings of the LDS Church to start with. That's the "milk," the stuff that's easiest to digest.
Only later, after a person has some level of commitment to and investment in Mormonism are they introduced to the more difficult, least justifiable, and most controversial Mormon beliefs. That's the "meat," the substance that's harder to digest and only given to a person after they have matured.
This is particularly true whenever questions are raised about whether Mormonism is a genuine form of Christianity or not. In that context Mormons will typically focus only on those aspects of LDS Church teachings which most closely adhere to traditional Christianity. All of the points on which Mormonism diverges from Christianity are ignored or dismissed in vague terms.
It's no coincidence, though, that once people have some personal investment in Mormonism, they are less likely to give it up simply because a teaching is difficult or appears irrational. Thus converts are only introduced to the "meat" of Mormonism once it's harder for them emotionally, psychologically, and socially to get out.
Sharing with Outsiders & Non-Mormons
The people with whom the least is shared about Mormon teachings and doctrines are probably outsiders who aren't prospective converts. At least prospective converts have shown a little interest in what Mormons believe and, if they are going to join the LDS Church, more and more will have to be shared with them.
Total outsiders, in contrast, don't technically have to know anything about Mormon teachings. Add to this the fact that Mormons have a long history of persecution at the hands of outsiders, most of it related to Mormonism's unique and controversial teachings, and it's only natural that people will develop very strong inclinations against sharing too much.
You can see this in the differences between what Mormons say amongst themselves and what they say publicly to outsiders. Mormon leaders will give speeches to Mormon audiences in which they reference doctrines like the idea that God is married and we all have a Heavenly Mother. You will not, however, ever see a prominent Mormon — especially if they are in politics, like Mitt Romney — mention that or other controversial teachings even when they are otherwise talking about religion generally and their own religion in particular.