Because we went with the reliability of a full Microsoft stack—.NET, C#, and MSSQL—our costs grew with the number of instances. Each server required a new license
They literally mention license costs as an architecture driving constraint.
When Jeff Atwood and Joel Spolsky started Stack Overflow they had a podcast. In that podcast Joel repeatably argued that hardware wasn't that expensive, compared to Jeffs time. His argument was that buying a larger server and scaling that way was much more cost-effective. I think that's still true, to a point, even if you didn't have the licensing cost.
I don't know if in the cloud world this still holds true, considering that (at least in AWS) if you vertical scale 2x you get usually a 2x price increase. Clearly I'm not factoring costs of making an app horizontally scalable or DB-shards aware (which I think was always the main selling point for SO vertical scaling: they usually referred to the DB, and they were against sharding)
As opposed to what? What reliability are they worried about trading off?
Don't get me wrong - I love C# and .NET, and MSSQL is a fantastic RDBMS. And if your people already are familiar with this tech stack (which I think is the case here), it makes sense to play to strengths.
But let's not pretend that this stack is the more reliable option to any other alternative...
Cannot comment on reliability, but considering .NET and MSSQL are from the same company, I'd assume they work great together.
Postgres was already a better choice than Oracle or SQL Server on almost every way (the clustering setup UX sucked). But very few people were aware of that, and they would have to learn a completely new stack to use it.
In our case we were also using MSAS and SSRS.
Anyway, the Microsoft implementation of data workflows is so stupid that you are better of with manual dblinks or creating the entire thing in a general purpose language. Their largest competitors share a lot of the same problems, so I'd say that workflow software is just an excuse to hire low-paid professionals and a real drag on any working development team.