What incentives does an advisor have to help PhD students graduate in a timely manner?
This is very simple. I care about my students, and want the best for them. Languishing in graduate school is rarely optimal for anyone. And then there is the issue of funding. For the same reason -- concern for my students' wellbeing -- it is very important to me to provide them with RA funding as close to year-around as possible. If funding starts to get tight, it doesn't help to have someone taking a luxury sixth year when he or she could have graduated earlier. I'd rather have everyone funded on RAships than have so-called "cheap labor" from senior students while junior students are forced to TA. 

If the question is referring to why even a sociopathic advisor would want his or her students to graduate on time, there is some prestige in having students graduate quickly and move on to good positions, and some shame in having students in their sixth, seventh, and later years.