From all I have seen, it lost because a significant number of French citizens do not want to allow non-French people to be able to work and take away jobs from French people. Since the EU Constitution basically allows open migration of citizens within the EU including working, the French saw this as a threat to their economic livelihood.
And some people are against it because they want to show Chirac that they don't like what his policies have done to their economy. These people will expect more socialism, even though more conservative economic policy is their only long-term hope out of the mess they are in (lower taxes, greatly reduced labor regulations, lower spending, more personal responsibility).
Seems to me that the above makes no sense. These are the same morons that want less and less hours in the workday while allowing unfettered immigration. Look at the Muslim population in France, what % even speaks French? No, I can't see the above as likely.