I think it's more likely that they simply haven't felt the need to update their styling much in the past decade or so. And last I checked the home page uses tables for layout, so it would make sense that it works on IE5, since that's what everyone did back then.
Re:Popular Websites Displayed in IE 5
by TazeTSchnitzel on Monday April 29, 2013 @07:28PM
(#5629530)
Actually, they did do a big styling change a year or two ago.
I feel like this is a more sufficient explanation than the one provided by the author. The front-end people at wikipedia probably remember the hoops they jumped through to work within IE5's quirks, and developed best practices that still happen to work out in that context.
This is awesome and most likely a part of their mission to make knowledge as accessible as possible. I have seen old, low-end donated PCs that are rife in elementary education in rural India and I'm sure in other developing countries as well. I am sure that these machines will be able to render wikipedia just fine!