Because a hackathon is about more than the final presentation. We've seen too many teams win without building anything more than a landing page and a "business model". This goes against the spirit of what a hackathon is, which is raw technical creativity. We want developers of all techincal level to come out and learn something new, break something, maybe even set something on fire (please don't though). The teams that walk away with prizes are those who came out the other side with something working, impressive and a weekend they can be proud of.
John David Back
Is your hack challenging? (40%)
Is your hack actually useful? Does your project solve a problem? (20%)
Is your project well designed and does it work well? (20%)
Is this a new idea, or is your project something we see at every other hackathon?