Do the wizards also gas their bananas? The choice of fruit here implies that wizards are bound to the limitations of modern farming and container shipping. The common prevalence of bananas in the real world boils down to a pile of technical limitations on growing and importing fruit.

1. Unlike most other common grocery store fruits they are tropical. Which means that they can be grown under normal conditions year round. It also means they need to be imported which would normally make them expensive
2. Bannanas can be picked green, shipped refrigerated, and ripened under ethylene gas (tomatoes and apples might also be in this category). This enables bannanas to be transported by ordinary container ships rather than needing to be done by air. This also enables some amount of stockpiling. This cuts costs a lot such that bannanas can be cheaper than local produce unlike most other imported produce that needs to be flown in.