The simple answer, which will sound pretty stupid, is that I decided early on that this would be a free game and so I felt ok only supporting the preferable control scheme. Normally supporting both is trivial, but for this game I put a good bit of work into the cool analog menu system, which I programmed for analog stick input only. I could theoretically make it work for keyboard, it just wouldn't be as quick a task as it would for other games where analog input & keyboard input are effectively the same.
I should really take a second look at the code though and see how much effort it would be to get keyboard working. I'll have a look in the next few weeks when I have some free time. If I release an update, I'll make a post about it so everyone knows. No promises though.