WHY when I log in whit my new iTunes ID my iPhone6 still ask me for log in whith my old ID ???

Hi,

This means your apps are still registered under your old apple id. In order to register them to the new one you have to remove and re-install the apps under the new apple id.

Unfortunately yes. Because the apps are not transferrable between Apple ID's unless you use family sharing.

Well it's a simple solution just a little time intensive. But once you re-install it under the new Apple ID you're good to go.