Hi Rob! 

The official name of the language is InterSystems ObjectScript or just ObjectScript - it's not only for Caché any more ;) 

And if so, why can't other languages be used instead?

Every contest has a topic. First one was about ObjectScript CLI. The current is about Intersystems IRIS REST API or web sockets interface. There is no ObjectScript requirement in this particular contest directly but it's obviously involved. Because if you even generate the REST API using spec-first approach you get endpoints but you need ObjectScript to write the logic. See examples in the article from @Eduard Lebedyuk  or another from @David Reche.

And next month we are having a Native API contest where applications using python, node.js, .NET, java are very welcome.