It seems (to me at least) as though orphans as significant or even leading characters in literature had a "heyday" from around the time of Dickens, through Twain and up to the Little Orphan Annie.

Yes, we have our share of current orphans in literature (Harry Potter, etc) but it seems less prevalent.
Possibly due to the amount of modern literature I read compared to that period I mention.
It may just be that the more "lasting" books from those times contain orphan characters.

Is this perception I have due to a higher incidence of orphans in those periods with reduced adult lifespans, or was it a response to their plight which authors of the time were drawing attention to?

Am I wrong and their prevalence has been fairly static throughout literature's history?