Software support is lacking
I wanted to try to use IBM in Cloud's free 200MB Db2 instance for something, but nothing I care for seems to use it. I guess I'm not really their target customer anyway, but I think they have missed the boat with respect to application developer mindshare.
The same for Oracle and their two 20GB free cloud databases. MediaWiki used to support it and MS SQL, but hasn't since MW 1.34 - and not well before that: https://mediawiki.org/wiki/Core_Platform_Team/Decisions_Architecture_Research_Documentation/Dropping_Abstract_Schema_Support_For_Oracle_and_MSSQL