Dumb luck. RNG is a fickle thing. Someone has to be the lucky one. lol
My first Raiku was 1840 something. Low. Lowest of anyone I have seen. My last one was 1890.
Even if you don't spend money you get a free raid pass a day. If you only raid Raiku you get one Raiku attempt a day for a month. It's going to be a very common pokemon. And odds are you should end up with a very good one.
I suspect, as in the other legends in my area, by the end of each legend's run near the end of each month you won't see nearly as many people trying for them. Once you have a good one, and maybe a few more for candy, you're only catching for reward items and XP. Which for hardcore players is almost more important than a decent Raiku (it is for me), but the groups of casual players drop off dramatically.
Still, just by the sheer number of opportunities to catch a Raiku vs. even Lugia (Raiku is apparently the only legend showing up at any legendary raids) the law of averages say you should get something in the 90's IV-wise.