RMau might have the best answer about people thinking they must register to actually be able to get the info they are looking for.
Here is a scenario that might help us to understand why people don’t participate on the forum. This is a story about me.

Once I went on a coin forum. I was just looking for an answer about a particular large cent variation for a particular date of coin. I did a search on Bing to find an answer to my question and I ended up with a forum. I didn’t join the forum or participated for the following reason.

1. Had to register and come up with a user name and password. I was in a hurry and was just surfing to look for an answer. Didn’t want to take the time.
2. There was rules that I was to read before I could join. Again I didn’t want to take the time.
3. I was supposed to introduce myself and tell something about myself. Again I didn’t want to take the time.
4. To ask a question I had to post in a special folder for the subject I wanted to ask. I didn’t want to take the time to figure out which folder and I dint want to be embarrassed if I messed up.
5. Just wanted my question answered and wasn’t planning on coming back to this forum.
6. Here is the Biggy, Even though I collected coins since I was ten, and I probably could impress any of you on my knowledge of coins, but the people on the coin forum was so much above me, that I was intimidate to participate because I felt so inferior to their knowledge.

I think any of the above could be happening with collectors with this forum. I think number 6 might be the biggest reason collectors don’t participate.