Stack Overflow existed for a couple of years before taking any funding and it was developed by three people who were friends (Jeff Atwood, Jarrod Dixon and Geof

Stack Overflow existed for a couple of years before taking any funding and it was developed by three people who were friends (Jeff Atwood, Jarrod Dixon and Geoff Dalgas) none of which were even getting paid regularly at the start and who were all primarily familiar with the .NET stack from previous jobs. It makes total sense why the tech is what it is. One could argue they could have shifted away from that stack since then, but one could also easily argue (very convincingly to anyone who has ever been involved with any significant project rewrite) "but... why?".