While I'm sure a bias towards the present is part of the problem, there's also the simple fact that it's easy to write about things plenty of other people are writing about, and about which details are plentiful. Many of the things Wikipedia is missing, but really should have if it wants to call itself an encyclopaedia with any pretense at seriousness are obscure, and their details locked away in obscurity. (As a consequence of this, many of those things Wikipedia does have articles about are couched in the language of someone who already understands the topic, mitigating their utility to those who don't.)

All that said, the article on the Republic of Ragusa was far better and longer than I'd have expected, given its use as your counter-point.