I thought the issue with Lard was the trans fats, but since I didn't know I double checked and apparently Lard has no trans fats, so I then wanted to know why we quit using it.

It seems that Lard was very popular up to WWII, but then we had supply issues due to the war effort so people switched over to vegetable shortenings, and afterwards pork fats like Lard were considered unhealthy compared to the "pure vegetable oils" which were often hydrogenated and filled with Trans Fats which were worse for you health-wise than Lard is.

So, yeah, WWII and food purity aesthetic might be responsible for some measurable portion of the modern obesity crises!