Why would someone who isn't from Philadelphia do this?
An announcement location is about crafting a narrative. Barack Obama announced his 2008 campaign at the Illinois State Capitol because he began his political career there as a state senator and it fit with his overarching message of, "Middle America is working even when Washington isn't."
Hillary's 2016 "kickoff event" (there was no announcement event since she announced via a YouTube video) was at the Four Freedoms Park on NYC's Roosevelt Island. It combines Hillary's adopted hometown and the state she represented in the Senate with an island and a park named after a president who shifted his party to the left and redefined Americans' "conventional" notions of what obligations the government has to people. It was a way to preempt the ascendant left wing of the Democratic Party at a time when, as with FDR, poverty and income inequality were major issues in public conversation.
Unless you can craft a narrative around Independence Hall, there's no compelling reason to use it for an announcement unless your campaign is being run by a fourth grader who skimmed through his social studies textbook.
In that case, why didn't Ted Cruz announce at IH? He was all "Constitution" this and "Declaraton" that and "Founding Ideals" that. While I agree with the sentiments, it is still odd he didn't announce there with this logic.