There are a few options for carrying cargo on a bicycle: saddle bags, a backpack, a basket on the handlebars. Big saddle bags are inconvenient and baskets at the front look lame. That leaves the backpack (uncomfortable) or the unsafe and less advisable option of sticking them in a carrying bag and slinging them on the handlebars.
If you take the last option, you have to fight for balance against the movement of the liquid – which is something you really don’t want to have to do when you are in traffic. So strapping them in the middle of the frame really makes a lot of sense.