Because she has been around for 28 years. See thats how senority works in the House of Representatives. Not too hard to get reelected 13 times in one of the most liberal districts of the most liberal state.
And news flash, just being a ranking member doesn’t mean you represent the party for example, Donald Trump had no senority or ranking in the GOP and still ended up the nominee. Roy Moore is a member of the GOP, does he represent the party as a whole? How about Joe Arpaio? Point being, just being a member of a party doesn’t mean you represent it’s entirety or in the case of extremists like Waters, even a significant portion of them.
You could say she represents ~700,000 people which is the population of the district that she serves, and that is generously assuming 100% of the people of that district are in the democrat party. Going by the last house race there about 75% of the district is, so bottom line is the best assertion that you could make is she represents 525,000 people of the democrat party, but most certainly not THE Democrat party.
