That's probably the biggest thing. Buying from Disney you know how the process will go, it goes quick, and your Sales Guide will help you get ressies in before you close.
Resales move slower. You have to find the resale. Make an offer. See if the offer is accepted. Perhaps counter offer. You can be prompt about mailing your paperwork back, but if the seller is less than motivated it will drag out. Wait for Disney to exercise their right of first refusal. If they do, start over. If they don't, now you start the closing process - moving paperwork back and forth again - once again, an unmotivated seller (or a seller outside the country) can make the process drag.
Resales are not for those planning their first trip in the next several months. They are better for people who have time on their side.
The other thing is financing. If you need to finance (and don't have an easy home equity line of credit) , Disney makes it easy, although not cheap. Resales are trickier (but still possible) to finance.
Finally, their is the value of banked/borrowed and spent points. Understand what points are on the contract you are buying. A spent contract may not be much of a bargain (by the time you pay closing costs, and figure Disney will sell you a spent contract via Magical Beginnings - you won't save much), a contract with available points may be a bargain - but only if there is a window available to use or rent the points.