Well, as much as I feel for Rain fans atm, I wouldn't hang the argument on the poll.
First of all Rain was just barely above Baraka and Kabal. And second, he was by far the least classic/integral to the franchise of the 4. So if you take a global, franchise sort of perspective to that poll, it's... not... surprising that this should be the outcome.
Well the flip side of that argument is that the second place is even more impressive if the other two choices had more prominent roles previously.
But the poll aside, sure. Rain isn't the most important character in the franchise's history. That's for certain. Yet there's a couple of things that make his absence feel unfortunate. There's the whole MKX disaster with him being a fightable NPC with a moveset but not playable. There's a picture in the menu that, at the very least, resembles him heavily. He's not in and maybe not the most popular for the vast, casual fan base, we get it. Yet it feels bad to see him sit out again after all of this.
He's been implied several times, he's been in the comics, he had a more important role in the story mode of MKX than many others, yet he never made base roster in the new gen games... It would have been nice to see him.
He was never implied in any way and is obviously not in. But after his popularity in MKX, I'm still surprised that they didn't include him again when I think about it. I thought he was among the more popular newcomers for sure.