It's simply because that's the stack we know and can utilize the best. We do have some non-MS stack things running but the stack we use is the one we know how to optimize and manage the best.
Really it's because the three guys who started it knew that stack the best. The rest of us are just hangers on :)
Thank you for the response. Usually on HN you dont hear a lot about MS products, its nice to hear about something different especially in a positive light.