AFAIK, it is because UPSERT are only a subpart of MERGE. The devs plan to add real MERGE in a later release (9.5 or 9.6, I don't remember). Someone more knowledgeable should shim in for the details.

The reason the author of the blog does not call it MERGE is probably to avoid confusion with the MERGE statement in the the SQL standard. Thew current plan is to add syntax to the INSERT statement in PostgreSQL 9.5 to ignore or return unique key violations. This is a subset of MERGE since MERGE does not require unique constraints to work.

For most OLTP users the proposed new syntax is more useful than MERGE since (unlike MERGE) it makes it clear exactly which rows will be locked. You do not want your application to stall on locks or give unique violations when doing upserts.

I don't believe there actually is an UPSERT command and that it's just a nickname for the activity. MERGE is a part of the standard, so I think any implementation would use that as the actual command.

I think the term UPSERT really gained popularity in NoSQL databases, such as MongoDB and CouchDB. That, combined with missing implementation details of MERGE could explain the term usage.