Why are indoor arenas called garden?

These arenas are dressed up on occasion, for different shows, because they look so austere most of the time, so for instance when they have a cat or dog show they will place flowers all around, or carpet the place for boats or car shows, not all arenas are treated this way, some are just 'arenas"

The Word Garden has many meanings, one being "a large hall for public entertainment". This is the real reason.