The 'sculpture garden' and the 'art gallery' are similar ideas.
For millennia, art was the handmaiden of religion. The idea of building a separate gallery in which to collect works of art is modern, in the sence of post-renaissance.
When art freed itself from religion, it became an adjunct to interior design. Pictures were placed on walls to please the owner, while also displying his wealth and taste. Sculptures were placed in gardens for similar reasons - and as an adjunct to garden design.
Abstract art, in the twentieth century, marked a further 'liberation' of art from its context. 'White box' galleries were built to make 'context free' locations for works of art. The advent of Installation Art is reversing this trend: artists and sculptors now wish to contextualize their art by 'installing' it in a specific location.
'Sculpture Gardens' and 'Sculpture Parks' developed as the outbox equivalents of 'White Box' galleries - and are also being affected by the advent of Installation Art.
The happiest outcome might be if the Sculpture Gardens disappeared - and sculpture returned to its roots, as a component of garden design.