why we didn't use the pronoun Y with future simple ?

Bonjour Ibrahim, Unfortunately, I cannot see the specific example you're referring to, therefore I'm struggling to answer your question. I can say that there's no reason not to use "y" in Le Futur Simple, when replacing "à + clause" or a location. I'd be happy to look at the specific case you've seen, if you can find it again. A bientôt !