People often avoid polyester when planning events because it is a synthetic fabric that doesn't breathe as well as natural fabrics like cotton or linen. This means that it can cause discomfort in warm weather or heated environments as it tends to retain heat and can make the wearer feel hot and sweaty. Additionally, polyester can have a slightly shiny appearance which may not be suitable for all types of events, especially those that require a more sophisticated or elegant aesthetic. Moreover, it is less environmentally friendly than natural fabrics, which is a factor that many people consider when making choices for their event planning. However, it's important to note that polyester is generally more durable and resistant to wrinkles and shrinking, making it a practical choice for some situations.