Note: The following is just my own opinion that's been forming for awhile, do not take this as a hit on anything, or other than what it is, my opinion.
After spending some time sitting back and taking a break from this site, to me part of the problem with adding experts here is being able to get people who can adapt to the culture of SE. My involvement with another research-based site shows me that there are two definite shifts in how questions/answers are presented in academic sites and places like SE where the community is more involved in how reputation and answers are perceived. Experts in some areas, and I can luckily say some of my history professors were not like this, can be prickly when their research and authority in subjects is attacked, which will happen in SE. I've seen it happen in many questions on SE History as a mod, though it's a small portion of what I've seen it may be a stumbling block with others getting involved. Especially considering the "burn" others may have had in previous exposures to SE sites and involvement in public forums like this.
Remember these are people who spend their time as experts in subjects and some do not take direct challenges well to their knowledge of subjects. While getting younger members of the community may help, as they are more familiar with the social media aspects of online communities and may have thicker skins for criticism that may arise. Introducing them to SE History may be good, but the introduction has to draw them in, and I am not sure even the current state of SE History questions is enough to bring them in. I know this goes back to the question quality problem we struggled with early on, and I can see it somewhat still happens, but then this is for people to have questions who may or may not be History professionals or academics, it's a place for answers. Consider as well, that while we have stressed in the past that answers should have some documentation for them, not all do. Many are someone's recollection or knowledge put out there, and some have sources to back up the statements, and many sources are not Wikipedia. What would be the expectation of an academic who is more used to using sources to back up their thesis or paper? This sort of loops back to the reputation aspect, here is someone who is already known in another community as knowledgable in their field but here they are starting over, many would not want to do that.
I'll stop there as I feel I've dumped enough. Thanks for reading!