The most common causes of no-shows are: the invitation didn't feel urgent or personal enough, there was no confirmation reminder just before the event, colleagues or managers saw it as not compulsory, the event sits in the calendar too far in advance so other things take priority, or there's no social pressure to come.
With corporate events, always count on 15 to 25 per cent drop-out of confirmed sign-ups. Reduce no-shows with a personal reminder, a teaser of the programme and a clear confirmation just before the day.