I haven't seen anyone saying the event was a failure. On the contrary, it clearly was a success. For the avoidance of doubt: bloody well done, good effort.

However, it could have been an even greater success, instead it failed to keep the interest of at least one person for long enough to get the message across. Why assume he was the only one? There may well have been others, and they too will have left early so you won't have had feedback from them. Steve has explained why it didn't work for him - the basic message is, don't assume your audience knows what it's all about, and get to the point sooner.

There are two possible reactions to this: one is to take his comments on board and consider whether future events might be arranged slightly differently to make sure the key message is delivered while you still have everyone's full attention.

The other, which with the exception of Eliza C seems to have been the reaction of most here, is to attack Steve for failing to understand a message you hadn't actually got around to delivering.   

Frankly, I find this attack on constructive criticism painfully reminiscent of left-wing student politics in my youth. This is feedback which should assist you to do better next time, rather than an attack on the entire event. For god's sake, don't be so defensive about accepting it as such.