Why is anything a "must see?" Because somebody put it on a list.
A lot of "must see" places have been turned into "don't go there, it has been ruined by crowding and tourist development." Because they were "must see."
My own must see places tend more to be urban places with different cultures, rather than ultrapopular parks, many of those not so popular for what is there, as they are for proximity to heavily populated areas where people need to go somewhere, anywhere, to get away a little bit, for a while. Thus GSMNP and Yosemite as the most crowded parks in the system, easiest to get to for large numbers of people.