I don't think they "needed" the Citus tool, per se, it was just the easiest option. I don't know much about MS-SQL, but no doubt PostgreSQL has areas that can be improved, or even that outright suck.
The main barrier against adopting MS-SQL is just the pricing and that it's not open source. Another thing that PostgreSQL seems to do a lot better than MS-SQL is in the extensibility department, hence we have things like TimescaleDB, Citus, EdgeDB, and a whole lot more. I can't really find anything like that for MS-SQL, but perhaps I missed it?