Just as the title says. I know, I've been using it that way for years, but I can't help but wonder why.

Rule of Cool? ;)

I'd guess, people are just used to an O in an initialism standing for "of" and therefor being lowercased as part of a title.  LoTR, etc.

Why is the "R" in "Toys 'R' Us" usually reversed?  It's just a style thing.

From a Wikipedia article on acronyms:

“While abbreviations typically exclude the initials of short function words (such as "and", "or", "of", or "to"), this is not always the case. (A similar set of words is sometimes left as lowercase in headers and publication titles.) Sometimes function words are included to make a pronounceable acronym, such as CORE (Congress of Racial Equality). Sometimes the letters representing these words are written in lower case, such as in the cases of "TfL" ("Transport for London") and LotR (Lord of the Rings); this usually occurs when the acronym represents a multi-word proper noun.“

I would assume that “on” from online is left as a lowercase due to the above.

Actually, no.  As I said, it's just a style thing.  Somebody typed it that way about 17 or 18 years ago (maybe longer), and it caught on.
This message was last edited by the user at 17:52, Sun 07 July 2019.

My two cents is...it is cool and I like it without trying to dig deep into it. *shrugs*

It is its own iconic trademark in a sense and I type it RPol whenever I use it anywhere.  Guess like LotR, etc.  and BBR is right it goes back years.  Ya know if someone started talking to me about roleplay online I think of it as a subject not a site. Say to me RPoL and because I have haunted these halls for years I know exactly right off what they mean.  Just a cool name...  for a Great site!