Because Formula E cars would not be able to turn with F1 sized tires?
F1 rules force very small rims. Not sure why, maybe to limit what sized brakes they can fit? Eitherway result are these balloon tires that also absorb shocks more than in other cars. (where suspension would do that more)
Formula E is slower so both these arguements do not apply so much anymore. Balloon tires are also very different to what is used in normal production cars so maybe that played a role, too.
Oh found a site that explains some reasons:
http://www.autosport.com/news/report.php/id/109795
"We designed them to be both energy-efficient and more like road tyres. As a consequence, the new FIA Formula E Championship will serve as a life-size laboratory that will help Michelin to prepare its tyres of the future."
...
"The decision is in line with the whole philosophy of the championship," he said. "Less rubber, less consumption, green tyres, and almost road tyres."