That guy was not rejected because his idea was bad. The prototypes he brought with were of very low build quality. The investors had doubts about his abilities of running the bussiness rather than the idea itself. I don't think they made a mistake.
A prototype is just to get an idea across. He easily could've hired a firm to design the product properly with the GBP100k he stood to gain from that contest.
The main motive behind these shows is entertainment. They just need to make a big showy production with drama, excitement, and hurt feelings. They are pretty lame imo. On the other hand, it's a chance for some investment, so I won't fault people for going after it.
