I think another potential explanation relates to the way people think about the history of a given field when asked to reflect on it (e.g. to create a top 100 list). We tend to conceive of fields as progressions unfolding over time, and even if we don't think this is always in a "better" direction, at least we conceive of the field as consisting of time periods characterized by a dominant paradigm or style. Certainly this is the way that "history of X" classes are usually taught.
If this is the case, it seems natural to me that, when asked to reflect on the "most important" individuals or contributions to a field in its history, we will tend to structure that reflection around our conception of these periods, and likely identify an emblematic individual for each period. Indeed, part of our conception of "greatest" might include a feature like "dominated their field for a decade or more," and obviously the frequency of individuals characterized in this way cannot increase with greater population, education, or anything at all! To the extent that our thinking follows this approach, we will tend to see "best of" lists being pretty flat over time, and therefore, appearing to decline when normalized by anything that increases over time.
I think another potential explanation relates to the way people think about the history of a given field when asked to reflect on it (e.g. to create a top 100 list). We tend to conceive of fields as progressions unfolding over time, and even if we don't think this is always in a "better" direction, at least we conceive of the field as consisting of time periods characterized by a dominant paradigm or style. Certainly this is the way that "history of X" classes are usually taught.
If this is the case, it seems natural to me that, when asked to reflect on the "most important" individuals or contributions to a field in its history, we will tend to structure that reflection around our conception of these periods, and likely identify an emblematic individual for each period. Indeed, part of our conception of "greatest" might include a feature like "dominated their field for a decade or more," and obviously the frequency of individuals characterized in this way cannot increase with greater population, education, or anything at all! To the extent that our thinking follows this approach, we will tend to see "best of" lists being pretty flat over time, and therefore, appearing to decline when normalized by anything that increases over time.