As I understand it, here are some of the reasons:
1. Close to a major city.
2. Plenty of land available right now.
3. Not close to any other major amusement park resorts.
4. Stable government.
5. History of low crime rates.
6. Decent climate, although not as perfect as some that were closer to the Mediterranean.
7. Close to major transportation arteries.
8. Close to existing major tourist attractions (Eifel Tower, Louvre, etc . . .)
9. Minimum of bribery needed to get it done.
10.Successfully partnered with an EPCOT pavilion.
During the 80's, when all of this was being planned, southern Europe was considered rather backward. If you wanted to get robbed, go to Spain or Italy. If you wanted broken machinery to stay broken, go to Italy or Greece. If you wanted to catch a 4 p.m. train in Italy, wander in around 5 p.m.
France was considered chic, and cutting edge. Fashion, style and food all came from France. They also had the fastest trains in Europe (the TGV), and their subway system was a model of clean efficiency. They were already the biggest destination of European tourists at the time.
I was in Europe as a student in 1985, and southern Europe was considered much riskier, with tales of tourists being robbed on trains while they slept, and beat up in dark alleys. France was clean, efficient, and beautiful. Yeah, the French were snooty, but everywhere else was either much colder, cloudier, or gloomier, or too far away.
You've also got to remember that the Berlin Wall didn't come down until 1990, and central Europe (especially Germany) was filled with American GI's and terrorist threats against them. And weather north and east of France was much worse. It was a pretty obvious choice back then. Today, I imagine Barcelona would be a first choice (in part because they put on a great Olympics in 1992), but in the 80's, Paris made the most sense.