They don't want the Harrowing to be just a Halloween event why so they can make more money. So the Harrowing we be in a few days then I bet you will see it again in 6 or 7 months
That's... not what it is at all? It's quite the opposite actually
I'm not saying it's good to not have Halloween-themed skins on actual Halloween (in fact I'm impatient for them right now), but they announced that they are separating Harrowing from Halloween for lore reasons, not because they want to jerk off some more money in march rather than october only. The Harrowing from now on won't be necessarily tied to skins, which means we could get actual deep Shadow Isles lore any time of the year without the compromise of making skins to go with it. They aren't ceasing the event skins, they just won't be forced to sell an event in order to get lore of a place in Runeterra.
tl;dr: make it more about lore, less about in-game content
Which also doesn't mean we can't get underworld (Shadow Isles inspired) skins any time of the year, even because we got Shurima themed skins (guardian of the sands) earlier this year without any Shurima event.
That's what the PBE boards announcement said (also posted on surrender@20). I don't know if it's misinformation or what, I see people complaining on the boards about how the lore is lacking but yet they seem to completely hate the idea of having Shadow Isles evolve lore-wise without being restrained to a specific week in October.