One possibility is that the IDEI is too free-market oriented for the French establishment. The French establishment, both on the left and on the right, is statist.
And of course there is a long French tradition of having the best of everything in Paris. A French academic who made it to one of the Paris institutes is not about to go into exile in the provinces. So if there is a large group of good economists in Paris, why not organize them into a school and gain critical mass?
There is now also a Toulouse School of Economics. It offers various programs, including a two year master’s degree program, taught in English.