> I can only assume that’s a testament to the quality of mediawiki.

I was curious about this so I poked around both and I think I disagree. Both load very fast for me and are snappy and look pretty nice. The one difference is that the Runescape wiki has a single ad in the sidebar or at the bottom, below the content footer. While the Fandom wikis have 3+ ads, far larger, one of which covers content until interacted with (like being closed). For me, Fandom's ad approach absolutely falls within "offensively bad," while the Runescape ad approach reminds me of early 2000s, "here's an ad to pay the bills. We've tried to keep it well out of your way."

So I'd opine that it has less to do with the quality of mediawiki, and more about how much money both Wiki hosts are seeking to gain from the existence of these resources.