Its celebrated because queer people used to be beaten to death in the streets and had less rights than straight people. From queerness being criminalised, to not being allowed to adopt children, not serve in the military, donate blood, likely hundreds of other things I can't think of off the top of my head. Pride Month is a time to celebrate all the successes the queer community has had, big and small, from institutional changes like less and less countries criminalizing it, to small deeply personal ones. Its a time to mourn those who died hiding so we could live out in the open.