Why don’t people keep resolutions?  A more useful question:

Why do people expect to keep resolutions?

A triumph of hope over experience.

The simple answer is that people don’t make resolutions.  Language channels make resolutions.  Keeping a resolution depends on other parts of the brain.  Those other parts may not buy into the resolution.  So they do their own thing and leave the language channel to go on flapping its lips. 

  I'm going to stop procrastinating.   As soon as I get around to it.