For example, copyright is self-executing. That means that you get a copyright in your program as soon as you write it. In contrast, a patent is obtained only after filing and prosecuting - and in the process spending a lot of money on - a patent application.
Moreover, copyright is supposed to protect a specific expression - not broader ideas found in that expression. One major argument against software patents is, that in order to maintain the lightning pace of software innovation, developers must to be free to use general concepts pioneered by others so long as they don't copy the actual code.