why all quest are xp?

We consolidated almost all rewards into one location, the Battlepass. As such, we tried to ensure that completing any Objective would grant progress along that Battlepass. It's up to you which you complete, where you focus your attention and how you earn your progress.

There are many 'Event based' Objectives coming in the near future that award other items and in addition to BXP, too (including a mission that awards our brand new champion: Mel). Look out for those in upcoming patches.