There's probably no technical reason why other than design choice. It may be related to cleaner URLs omitting the %20 encoding.

However, hyphens would probably have been a better choice for URLs as Google treats them as individual words as opposed to one long keyword.

Also, hyphens save us an additional <shift>-key press. Hardly on topic, but my advise is to require underscores only when there is absolutely no alternative. Which is almost never the case.

The forced .md and underscores was mostly for speed of development on the first version of Daux. I agree it is a bit arbitrary and I plan on making the requirements a bit less strict in the future.