From what I remember reading when I was frequenting the bay12 forums, it's a mixture of pride (the code is very rough, and it would probably take a huge amount of effort to get it into a form where contributions from others could be useful, or even where others could contribute) and control (he doesn't want it to get out of his control, and as it's his only source of income he doesn't want to deal with donations and community being fractured between several versions). The fanbase both doesn't care (based on donations) and sees it as a challenge (there have been some interesting analyses posted of what the game is doing, and there's a thriving modding community - both in the form of modifications to the game's files, which is supported and encouraged, and modifications to the game through memory editing, leading to the creation of invaluable tools like Dwarf Therapist).

If your only reason for not trying it is that it's not in apt, just go to the website and download it. It doesn't take any installation; just download it, extract the contents, and run the game.