It's a pricing problem. To begin with, $25 probably isn't enough to make the project financially successful, as it can't be expected to sell in large volume. Small-volume products need to find a sweet spot in the demand*unit-price curve. If no such spot exists, as may well be the case for software development tools, the developer may as well give it away. The Mac market is particularly low-cost, low-return.
Secondly, if the product were priced at a point where it produced a viable revenue stream - for example, Logic Pro X addresses a very large market and is priced at $199.99 - one could hardly expect the student discounted price for the full product to be less than $25. However, my experience has been that no matter how low-priced the product (greater than $0), students will always ask for a lower price. :)
It's sold through the Mac App Store only right now, so it's limited to what the Mac App Store allows, unfortunately.
Maybe in the future, if it goes well, the developer might consider selling outside the Mac App Store, and this have more flexibility. I have no knowledge of his plans on that matte,r though.
Been a while since I looked at App Store policies, but couldn't they charge a discounted price outside the store and then give a promo code to the app?
I believe that it's not intended to be used like that (promo codes are also limited to 100 per application version now).
Also, this would require the developer to find a way to charge people outside of the store. In that case, it's almost half way to setting up a full direct store with a direct version.
There is benefit to having a direct version (all our applications do) but many developers today don't want to set this all up. It's quite a lot of work and perhaps the developer is not interesting in spending time on that. Moreover, he might not even be sure if the app is successful enough to invest time into this work.
Looks pretty cool though.