The SQL Server query planner is head and shoulders above what Postgres offers in the types of optimizations it will apply to your queries. It also properly caches query plans.

It offers heap tables, as well as index organized tables depending on what you need.

The protocol supports running multiple queries and getting multiple resultsets back at once saving some round-trips and resources.

Also supports things like global temp tables, and in memory tables, which are helpful for some use cases.

The parallelism story for a single query is still stronger with SQL Server.

I'm sure I could think of more, but it's been a few years since I've used it myself and I've forgotten a bit.

It is a good database. I just wouldn't use it for my startup. I could never justify that license cost, and how it restricts how you design your infrastructure due to the cost and license terms.