I suppose its because people would have had to host a C# Wordpress on Windows servers instead of nice cheap Linux ones. Maybe that will change going forwards with .NET Core etc
And not just the expensive windows server but for a long time people considered windows builds of postgres / mysql not production-ready (How much of that was fud and how much truth I do not know), so typically any DB solution is Sql-Server which is another significant expense.
They still do.
We have customers running Linux instances just for Postgres.