I haven’t been to WDW in years (two decades to be exact) but is RoA that much more unpopular there?
I mean in general I feel like the rivers here at DL capture a good portion of that theoretical capacity with the canoes, rafts and the two vessels usually filled at a near full capacity at least.
Fantasmic and the New Orleans Square Train station are particularly crowd heavy locations that have attractions also optimizing the river.
There's much less traffic on the river for a variety of reasons: the same sort of decline in options over the years that Disneyland suffered, plus the loss of the canoes (for everyone except the cast member canoe races, apparently) and the early accident that killed the
Joe Fowler (their Mark Twain).
The attractions run limited hours (though I can't say offhand how similar or different their operating schedules are when compared with their DL counterparts), and the comparative lack of river traffic probably contributes to their lower ridership; you're less likely to want to ride something on the river if you don't see something on the river for long stretches of time.
There is no Fantasmic shown in that location to incentivize the keeping of the river or the riverboat, and the river in general feels slightly more removed from the midway than it does in California (though not removed to the extent of Tokyo, where I could easily picture someone visiting and not realizing there's a river there at all).
And a not insignificant factor is the uniquely intense pressure on WDW guests to maximize their vacation and do all the big cool stuff, exacerbated by FP+ and subsequent skip-the-queue systems, which doesn't leave people feeling like they have time to give to any of the river stuff, especially when combined with on-average longer queues, more punishing weather that leads more people to throw in the towel earlier than they might at DL, and shorter operating hours.
All of these factors undoubtedly play a role in the comparative lack of popularity of the river attractions over there.