If you are the presenter in a professional setting, your priority is making the information or data you are presenting the main focus of attention. The object is for your audience of potential clients to notice and remember the content of your presentation, not the clothing you are wearing or the background appearance of the stage/display area. For this reason, clothing and colors/fabrics/furnishings not directly associated with the product, service, or idea being presented and promoted should be as understated as possible. The goal is for everything not being marketed to fade into the background, while the material being presented is highlighted and made to be appealing and memorable.
Because too many colors may become distracting and prevent the audience from being able to see your presentation or hinder them from being able to focus on what you are saying. Having your presentation professional helps people stay on task and helps get your point across.