Because hardly any one knows it even exists and it's smaller and better than pretty much every other AV (inc Avast). I used to use Avast but now there's no point. It constantly nags you about upgrading to the latest version then breaks when you do.
It's rare that M$ get something right but MSE is excellent.
It constantly nags you about upgrading to the latest version then breaks when you do.
Weird. It's been the most stable, least naggy thing I've probably ever used. In fact it's so un-naggy that being hidden in my systray I totally forget I'm running it.