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Because MS-SQL offers features not available with MongoDB

True but,
At present all big big company use MongoDB,
Exmp: Facebook

Facebook (And any other company worth their salt) uses MongoDB where it's a good fit, and Postgres/MySQL/OracleDB where they're a good fit.

Steemdata is a mongodb database, however, it's no-where near as flexible as a standard SQL database for this kind of thing.