Why would a buttercream icing not be suitable for a cake served outdoors on a hot day?

Buttercream icing is not suitable for a cake served outdoors on a hot day primarily because it melts quickly in heat. Buttercream is made primarily from butter, which has a relatively low melting point. When the temperature rises, the fat in the buttercream can soften and lose its structural integrity, leading to a runny consistency that does not hold well on the cake. This can result in a less appealing appearance and a messy dessert that is difficult to serve. Additionally, the creaminess of buttercream can denigrate further under high temperatures, making it a less reliable choice for outdoor events, especially in warm climates.

While options related to taste or difficulty in application may have their own merits in different contexts, they do not address the immediate concern of maintaining the icing's structure and aesthetic in the heat, which is crucial for a successful presentation at an outdoor event.