In modern times (since 1980), this is likely because a Royal Flush pays substantially higher in video poker than a Straight Flush. While payouts vary by machine, it's common for a Royal Flush to pay 800 - 1000 times the stake, where a (non-royal) Straight Flush only pays in 40 - 90 times the stake.
For player-on-player poker variants, a Royal Flush is not actually a different hand category from a Straight Flush, and many hand ranking lists (such as Wikipedia's) do not list it separately.
Poker is full of slang; there are an especially high number of poker terms for various "best hands". The best hand possible in a particular situation (such as given a set of community cards) is kowns as "the nuts". A pair of aces as hole cards in Texas Hold'em is the statistically best pre-flop hand, and so has a whole pile of nicknames (Rockets, American Airlines, Bullets, etc.). An Ace through 5 straight is the best hand in some variants of lowball poker, and so has picked up the nickname the "Wheel". Naturally, poker has a nickname for the best hand in the game as well, i.e. the Royal Flush.
Merriam-Webster lists the term Royal Flush as originating around 1868, but doesn't list a source. However, it is worth noting that the term "Royal Flush" is prevalent enough to have made it into Merriam-Webster, where terms like "wheel" or "rockets" are not. Given this, I would speculate that some hand orderings list Royal Flush as a separate hand category from a Straight Flush to prevent confusion on behalf of the players who are aware of the Royal Flush as a poker hand (and thus expecting it to be listed in a hand ordering).