The applicability here is fuzzy. The author even stated:
Though in general, I would consider my data in this particular set to be more trustworthy when it comes to comparisons between fetishes as opposed to absolute prevalence of fetishes.
Though looking at the methodology used, its probably about as good as anything. However, I would object to the classification of many of the included topics as "fetishes" in the normal use of the term. You could easily have called them "random shit having to do with sex". Remove those and the distribution and scale shifts.
The chart's relatively low interest in feet does kind of fly in the face of my knowledge. Granted, I don't keep up with "the literature" but up til now everything I had seen had placed foot fetish (and related clothing) near or at the top of prevalence.
This got me thinking that maybe foot fetishism has "aged out" in this world of Western sexual liberalism. Similar to how hair and ankles were so sexualized in certain past segments that groups/religions forced women to cover them (I'm referring to Western stuff like Catholics and early Protestants--not Muslims). Maybe whatever nature/nuture affects that inspired foot fetish in the past isn't that much of a thing in tech-friendly modern Western cultures.
Though I think, in the end, that the answer to the OP's original question is that its a combination of aggressiveness of customer requests and of the producers' own relative attraction to feet. We've all seen how a small very vocal minority can effect change, so 2 guys screaming "FEET!" all the time could inspire creators. The producers have to be willing to take that risk though and that will be impacted by their own feeling toward feet. A relevant example would be one specific producer's obsession with armpits.
And missionary is just one of the easiest positions to fuck in with easy access to feet.