Why is a right pattern the standard for a hold?
This is a total shot in the dark. And, I have no citations for this. I always assumed that the a left-hand traffic pattern and a right-hand hold for a missed approach put the aircraft on the same side of the extended runway centerline. Presumably, this would be a more “protected” side of the airspace. Since standard traffic patterns for fixed wing aircraft are left, that would make the holds to the right. 

Although, AOPA says the following...


  “Many holding patterns are executed in VFR conditions, which means that those in a holding pattern could encounter opposite direction VFR traffic that might be climbing or descending through the holding altitude. Veering to the right (as required by right-of-way regulations) to avoid opposite-direction traffic keeps the holding aircraft in its pattern. This would not be true when executing a left-hand hold, which is why left patterns are the exception, established only when necessary (because of terrain, for example).”
  https://www.aopa.org/news-and-media/all-news/2017/may/01/ifr-fix-what-were-they-thinking