why are only 2 of last twelve questions open?
Every question put on hold has a close reason displayed in yellow. Generally, they also have comments because the generic close reasons can be unclear.
So, you can see for yourself why each one is closed.
Seems like they're closed when people here don't know how to answer the question
That's not a close reason; none were closed for that reason. (Although, a question that cannot be answered with scientific skepticism, or is better answered on a site with subject matter experts, might be closed as off-topic - e.g. this one)
like the vaccine one
The vaccine question didn't have a notable claim. It was a rather confusing question, and you will notice there has been some effort to work out how to best re-word it, both in the comments and in a chat room.
Arguably, it isn't a lack of expertise, but a wealth of expertise that is slowing this one down, as people dig through the anti-vax position (not held by the OP!), trying to find a clear claim that can be supported or disproved. Knocking down a strawman is of little value.
I recommend reading the Welcome to Skeptics meta-question, which is widely linked in the comments on posts by new users.