Some pasts are more lost than others
Just in case you didn't believe me when I said the past is lost, go read How women disappear from history on Philobiblon. Natalie Bennett provides a concrete example of the phenomenon that I described in rather abstract and evidence-free terms. This sort of thing (the "forgetting" of apparently important bits of information) happens far more than most people realize.
Now, you could argue that when it comes to the important stuff, like politics, we've got loads of information from well-preserved sources. There's no chance, after all, of us forgetting who the first president of the United States was, is there. And you'd be right. But you'd also be privileging political history as the "important stuff." To be sure, understanding the history of politics is crucial to creating any comprehensive outline of the past, but it's hardly the only thing that matters. There's a whole lot more to the past than elections, cabinets, and bills.
Certain types of history, then, are particularly susceptible to the problems outlined in my previous essay. While I'm sure there are plenty of sources political historians wish they had, their source material is typically a whole lot more obvious than that of social or cultural historians. Governments produce lots of paperwork. Not all of it gets preserved, but a whole lot of it does, providing plenty of fodder for later political historians.
The preservation of sources is not as haphazard as my earlier essay may have suggested. People choose to preserve those documents they find most interesting or valuable. The very selection of documents and sources to be preserved and archives, then, reflects the values of the time, values that do not always match the interests of later scholars.
Now this doesn't mean that Bennett's right, that the female founders of the charitable organization in question were forgotten because they were women. It's possible that charitable organizations as a whole didn't keep good records on their founding. And it's also plausible that the records for this particular organization just got lost somehow, whereas the majority of such female-led organizations did maintain these records. It's just impossible to know without further research.
Still, it's important for historians to remember that many of the sources they use are there for a reason, and many of the sources they don't use (simply because they no longer exist) aren't around for a reason, too.