[SahAssar]

I don't know the reasoning being not doing it, but I'd guess it's related to those DB engines not being geared or interested in embedded use-cases and that no SQL engine seems to entirely agree with any other how to handle certain parts of SQL.

[mschuster91]

Indeed, it would have been a lot of work trimming down any of the major FOSS RDBMS towards embedding them in a browser.

Regarding the SQL dialect - that could have been handled by specifying a standard SQL dialect that's then dynamically translated to the target engine's dialetc.