My patient contacted me recently to ask me why every single denture of hers gets very dirty.
I decided to ask me Dental prosthetist friend colleagues and we did a bit of brainstorming to find the right answers. Some of them were pretty obvious, some other quite interesting. And for all of you who would like to investigate in more details, there is a link to a very detailed study on the bottom here.

Too much coffee or tea (but sometimes even one cup of tea or coffee can contribute to this problem)
Smoking
Saliva flow affected by the medicines taken such as for HBP
Too much tumeric
Toothpaste or abrasive toothbrush that scratches the denture surface and destroys high polish
Mouth breathing (causing dry mouth)
Bad finish of the denture - gingival junctions not smooth enough