I would assume it's a difference in methodology and intent. Zouken's talents lie in doing spells where the effects return to him (Sakura says she can't use curses because they would rebound on her due to the nature of the Matou magecraft) and which drain people, so it makes sense that his method of staying alive would be to live off other people's life force.
Unfortunately (for him), though, doing so causes the soul to rot, because it loses the connection to the original body. Essentially, Zouken's soul keeps having to over-write new bodies with the "blueprints" of his original one, and over time those blueprints become corrupted and the body decays faster and faster. Other magi can seemingly live a lot longer without that issue because they don't survive by stealing bodies from others. I think it's pretty rare to have a magus that lives as long as Zouken, though.