Looks like there's a thorough discussion of the pros and cons on the PostgreSQL wiki here: https://wiki.postgresql.org/wiki/UPSERT#SQL_MERGE_syntax

The semantics are slightly different - the Postgres command doesn't do the exact same thing. So rather have two identically named commands that do slightly different things, they picked a different syntax so it would be clear they aren't the same.

Some of the reasons were discussed previously: https://news.ycombinator.com/item?id=9509870