A number of reasons lie behind it:
Money (or lack of)
Me hating attention
CBA with organizing it
We already have kids and have been engaged for a while
I'm pregnant
Our families' are not your typical 2.4 nuclear sort
SIL is having a big wedding doo next year
A typical wedding is a long day that I could never enjoy whilst running around after 2 kids
I hate other peoples' weddings and frequently decline invites/escape early
I know people are going to get a cob on with us but does it really matter? Should the day just be about getting married, declaring our love for one another bla bla bla or should it have to include family and friends who have shared it with us along the way?