Ashton Kutcher has finally pulled the trigger, filing for divorce from Demi Moore ... over a year after the couple split up ... and now we know why he waited.

34-year-old Ashton filed the docs today in L.A. County Superior Court, citing irreconcilable differences. He uses his full legal name 'Christopher A. Kutcher.'

Sources familiar with the situation tell TMZ ... Ashton waited so long because he wanted to give Demi the chance to file first ... it was a whole dignity thing. After a year ... when Demi showed no signs of making her move, Ashton finally did.

Ashton and 50-year-old Demi have no children together. They were married in September 2005 and separated in November 2011.

According to the docs, the property rights of Ashton and Demi will be worked out later.

Ashton and Demi split in the wake of cheating allegations involving Ashton and another woman. Ashton is currently dating his "That '70s Show" co-star Mila Kunis.