It's syntactically more straightforward to resolve the issue of clashing definitions through namespacing with functions compared to operators.

The following is pretty standard:

    foo::plus(a, b) // or foo.plus(a, b)
    bar::plus(a, b) // or bar.plus(a, b)

Whereas this is more awkward:

    a foo::+ b // or a foo.+ b
    a bar::+ b // or a bar.+ b