One reason was food reform. By the early 20th century, new knowledge about nutrition science, combined with an obsessive (but misinformed) interest in digestion, fueled widespread “expert” condemnation of dishes featuring a range of ingredients mixed together. This was due, in large part, to xenophobia; by then, many white Americans had come to associate mixed foods with immigrants.
Instead, reformers insisted with great confidence (but scant evidence) that it was healthier to eat simple foods with few ingredients: meals where meats and plain vegetables were clearly separated. People started to view savory puddings as both unhealthy and old-fashioned.