参加させて頂きました means you were happy to attend (showing respect and appreciation) while 参加させられました means you were forced to attend. As Vivian mentioned, 頂くshows respect. So させていただく means to happily do something, 見せていただく means someone (usually higher level or someone not close to you) shows you something and you happily see it. On the other hand, させられる is used when you are forced to do something. (i.e. 食べさせられる (forced to eat)、行かされる (forced to go) etc.)

there's a difference between them. itadaku can mean that you respect the person who asked you to participate.