Well, the reason is simple: there is more to gain than to lose by NOT disfellowshipping. If they did disfellowship everyone who married "out of the truth", there is a high chance that such ones WON'T be coming back and the org would lose them forever. So why not take the softer approach - make it permissible, with the hope that the believing one can eventually bring their mate AND any children that they may have into "the truth!" This would be an acceptable solution for many single ones (especially sisters) who are not able to find a marriage mate within the org. So while the org considers marrying a "worldly person" to be wrong, they still allow it!
Actually, I think the reasons are even simpler. They can get sued to their wazoos if they disfellowship people who marry whoever they decide to marry and get disfellowshipped for it. Marriage is a legally binding contract too, protected by the state. People in USA obtain over 1100 government benefits, entitlements and legal protections simply by getting married, and people cannot discriminate based on who they decide to marry.