Waterdeep has way more attractions and interesting things going on for the most part then BG. I mean 5e had to add a bunch of new districts outside BG's walla to make it more interesting.
Walking statues, mermaids in the habour, over three times the temples, robotic barkeeps, a mythal, archwizards, magic school, Mirt the Moneylender, and so much more.
Plus Undermountain.