John Du Hart wrote:

I really don't understand why we'd rather suffer than use a superior proprietary product.

David Gerard pointed out:

https://wikimediafoundation.org/wiki/Values

Duplicability down to the infrastructure is considered extremely important, or the free content isn't free. "Open core" fails this test.

Not to mention open source allows WMF and volunteers to fix bugs and add features themselves. Non-FOSS software also greatly decreases the chance of available volunteer expertise and help (unless the software happens to be extraordinarily popular (e.g. Windows, Word).