I understand that... but would have assumed they would have changed over time. Plenty of platforms have migrated backends to handle much larger load (twitter, facebook, etc, etc).

Wikimedia's resources are over two orders of magnitude smaller than those of the other top-10 web sites. It is only fairly recently that they have more than a handful paid developers. And they have a huge amount of data they would have to convert. In fact, I think they would have been up to it, but there simply were more pressing concerns.