Why is 'super' a keyword rather than a method in Ruby?

In Ruby, super is a keyword rather than a method.

Why was it designed this way?

Ruby's design tends toward implementing as much as possible as methods; keywords are usually reserved for language features that have their own grammar rules. super, however, looks and acts like a method call.

(I know it would be cumbersome to implement super in pure Ruby, since it would have to parse the method name out of caller, or use a trace_func. This alone wouldn't prevent it from being a method, because plenty of Kernel's methods are not implemented in pure Ruby.)