While the system can handle any situation with a quick judgement the rules as written appear to break down in long term play. This isn't hard to change, and the Black Hack begs to bent to the whim of the table, but I figure for this question I should stick to the rules as written.
Part of the reason it's so good for a short campaign is character generation. Characters can easily be created in five to fifteen minutes so there is no reason you can't get right into the action in the first session. With time limited in the campaign you don't want to waste too much rolling characters.
Another point in its favour is the speed of the system. At 20 pages it's about as rules-light as you can go with D20 game. There is no time lost looking up rules, and there are no fiddly bits to slow down the players. They declare and action and test against a stat for success. That's it. This allows the group to cover a lot of ground in whatever adventure they are playing.
The advancement system is loose and can be as fast as one level per session. That means the characters end the campaign at level 10, the old name level from the original fantasy RPG. This means in only 10 sessions the PCs can develop all the way from zero to hero. That short development is something you aren't going to get with another traditional RPG rules as written.
The system is adaptable. You can use it with the list of monsters in the back to put together your own adventure or adapt on the fly anything made for the D&D family of systems. What it comes down to is the Black Hack is an easy choice for a short campaign, whatever your goal.