Lille Flandres is a terminal station: the tracks end there and trains have to reverse and go out the way they came in. Lille Europe is on a through line used by trains from Brussels to London, and from Brussels to the south of France; if these trains used Lille Flandres, they'd have to change direction and their journeys would take longer.

bkmk

Though this thread was from 2008 reviewing it most of the info seems to be about the same validity in 2011.