> Anybody can explain why SQL database makes sense for large time series data?

Let's say, just to say, that you have effectively free licensing for Microsoft SQL Server (because you already own the licenses in question), and you have staff who know that platform well.

Sometimes it's easier to use the tools you already have rather than go acquire a new platform to achieve a goal.

As with anytime you choose a persistence layer, you're accumulating technical debt the instant you make the choice. Over time, if better persistence layers become available, you have to make a judgment call about when it makes sense to change the code, versus when it makes sense to do maintenance work on the persistence layer.