It's mainly because I'm not the Bible-answer-man.
I used to be. I used to answer any question asked in class because I feel like I had to. Wasn't I the expert?
No, I wasn't.
Here's what I do nowadays. If you ask me a question (either in class or via email), I might just answer it. On the other hand, I will often answer it only if I've written on the subject. If I haven't written on it, I will likely say something like this: "That's a great question, but to be honest with you, I've never really given it the thought it deserves. On the one hand, I could give you all the stock answers I'm familiar with. But it would save both of us time if you just read a good book or essay on the subject. Here are a few resources to get you started."
Let's say you asked me how I interpreted Paul's highly debated "husband of one wife" requirement in 1 Tim. 3:2. Well, I could point out to you the major views on the subject. But I probably would say, "I can't give you a definitive answer because I haven't worked through the issue in enough detail to give you an opinion I'm happy with. But here's an excellent essay (or book) that contains the four most common views on the issue." It's not that I don't have personal convictions on many issues like this one. I do. But I'd rather you worked it out for yourself in consultation with the experts on the subject. I am definitely not an expert on every exegetical debate in New Testament studies!
On the other hand, there are quite a number of important topics I have written about. In that case, I am only too happy to share with you my opinions/convictions. After all, I worked very hard to form them. I will usually give you a link to one of my books or essays and say, "Have at look at this." Even then I will add, "It's only my opinion, of course. You will still need to read what others have said."
I believe there comes a time when we have to become our own teacher, educator, schoolmaster, coach, pedagogue, even counselor. This does not mean that we don't study what others have said or written. Of course we do. On the other hand, there is such a thing as a healthy distrust of experts. Most of us go to school (even grad school) to learn what others have thought about this or that. After graduation we continue this practice. We become parrots, mere imitators of what others think, without having our own convictions based on our own research.
That is why I write books, essays, and even blog posts. I write not so much to tell you what I think but to tell myself. Writing allows me to find out what I think and to discover what I believe. Unless we do that, we don't really know what to think.
Thankfully, I ran across a number of authors in seminary and grad school who were original thinkers. I remember Dr. ____________. His thoughts were his own. I couldn't put his books down. Today, I read very few authors. But I read him again and again and again. Authors like him have earned my trust because their writings have substance. It's impossible to learn, really, without reading geniuses like these. But they would be the first to tell you to put your own thinking cap on.
And that is why I might not answer your questions.
Then again, I might!