I can't really give a good "why" answer to this, but I can give a "when" answer. In the early Federal period in United States, common cookery as fundamentally similar to the English foods of the late 18th and very earliest 19th century -- savory pies, lots of pottages, breads, and meats are mostly roasted or boiled.

Indeed the rich mince pie, made with not only suet but actually ground beef and sometimes venision, remained a common banquet or holiday meal in the United States right up into the 1940s. There was a great article in the Chicago Reader a few years back on this very subject: https://www.chicagoreader.com/chicago/mince-pie-the-real-american-pie/Content?oid=1267308

The short answer is that at this time nobody quite seems sure exactly why mince pie and other fatty savory meat pies declined greatly in popularity in the United States, but it was definitely some time in the first half of the 20th century that that the change took place. In newpapers and magazines of the early 1900s, mince pie and other savory pies are very much still popular, but by 1940 they are increasingly rare. It's not exactly clear yet why this change took place, but I'm hoping some intrepid researcher will investigate this mystery.

Greetings!

Before you comment, please take note of the rules of this subreddit. They state quite explicitly that answer should be in-depth, comprehensive, and of the level a historian would provide – this also applies to corrections of the premise of a question.

Meaning that a link to a supermarket in the US that carries savory pies is decidedly not enough as is posting "Chicken Pot Pie".

Please make sure to adhere to the rules posted above.

Thank you!