Atlantic is catering to people on the Atlantic ocean, thus to people in the U.S. East coast, perhaps also in the mideast, in Eastern Canada, as well as those in Europe at the other side of Atlantic, all of whom have a playable ping there. (I think this is because of the transatlantic submarine cable connecting North America and Western Europe).
Pacific, on the other hand, was once also one of the most popular shards, but it only covers areas such as California and Western Canada. The U.S. midwest also is less populated than the mideast while on the other side of the Pacific, the Asian-Pacific nations got their own (still populated) servers such as Formosa or Asuka. Also, South Americans were not rich enough to pay the 10 dollar subscription fee back in the day, meaning that these people often went to free shards instead to Pacific or other North American shards. So on Atlantic, you cover two continents, while on the other servers, you cover only 1 continent and often less populated areas within the U.S.
Furthermore, if you take a look here:
https://surpricebox.com/s/2019/08/p...ight-savings-clock-missouri-maine-970x469.jpg, you see that the Eastern time zone is home to the vast majority of people living in the U.S.. Thus, Atlantic represents the underlying demographics of the general population as well as maximizes the player base for a given time zone, so that players can hold guild events, participate in EM events, engage in PKing etc. at the same time when everybody else is online.
As others have posted, numbers attract numbers, so nowadays players with a high ping or from completely different time zones, not to mention countless "shoppers", moved to Atlantic (snowball effect).
Lastly, Atlantic was also one of the original shards, and its name is simply easier to remember than the word "chesapeake" or so, which the average 12 year old player signing up for UO in the past might have never heard of.