I can think of several:
1. Monopolistic lock-in. DRM is more than often used to control the market. It happened with Apple in the past, and was one of the key reasons that music publishers realized that being DRM-free is actually better for them.
This reason also includes DRM derivatives like DMCA-1201 and the like. It's all about control (over the markets, over users and etc.).
2. Covering one's incompetence. DRM is used to justify failing sales (i.e. when execs are questioned about why the product performs poorly, they say "Pirates! But worry not - we put more DRM in place").
3. Ignorance and / or stupidity (many execs have no clue and might believe that DRM actually provides some benefit). This type can be called DRM Lysenkoism.
None of these reasons are valid, all of them are crooked and anti-user, but they are often present in various combinations.