Aside from stray threads of apparent rampant toxicity about the topic, I think the main reason people hated G2 is because it wasn't a ten-year-long story-heavy expedition into adventures beyond description and lore so deep you could drown the Skyrim wikipedia in it.
And that's really, really petty.
You have to understand that ten years is a good while. You don't condense the work of a decade into a year and a half. Let's compare the first two years of G1 with the entire run of G2 to make this a little more fair.
G1's Plot: Six heroes arrive on the island of Mata Nui from an ancient prophecy. Meeting up, they join forces to take on the minions of the evil Makuta, who wants to establish his rule through the misery of the inhabitants of the island. They defeat the source of his minions, which is Makuta himself, before facing an unstoppable threat in the form of the Bohrok. barely overcoming this new foe, they become Toa Nuva, even stronger than before.
G2's Plot: Six heroes arrive on the island of Okoto from an ancient prophecy. Meeting up, they join forces to take on the minions of the evil Makuta, who wants to establish his rule by finding some way to return to the island. They defeat the source of his minions, which is Kulta, the Skull Grinder, before facing an intimidating emissary from Makuta named Umarak the Hunter. Finding themselves overwhelmed, they take on Umarak's transformed version at the height of Makuta's power before sacrificing themselves to seal him away forever.
Now this isn't so much a fair comparison so instead let's compare a couple sets from the first year: Lewa and Lewa: Master of Jungle.
Lewa (G1): Off the top of my head the figure comes with 34 pieces? I'm probably wrong but it's somewhere close. He retailed for $9.99 USD. He has a gear function which swings his right arm, limited poseability, is extremely similar in all aspects to the rest of the Toa in that wave being a near copy of them, and very little in terms of MOCing potential. His primary colours are green and lime green, with some black and light grey thrown in as well.
Lewa, Master of Jungle (G2): I don't have the parts list with me but I think it's somewhere in the one hundred range. He retailed for $14.99 USD, five dollars more than his grandpappy. He has a gear function which swings both his arms, great poseability, a weapon repurposing similar to the Toa Nuva, is rather distinct from his Toa cohorts in that wave, and has a fair bit of potential in MOCing. His primary colours are that newfangled green you kids are all crazy about these days according to LEGO, keetorange, and silver. I don't know about you, but I know which one I'd be saving my tooth fairy money for.
Now there's no way anyone could possibly justify G2's very loosely slapped-together story as being somehow on-par or better than G1's, even in the first two years. But people don't seem to want Bionicle, even if the sets are objectively better and reasonably fair with inflation in mind. No, people want a ten-year-long series of epics, preferably piggybacking as much as possible off the original. Think about it: if you hate G2, suppose G1 never happened. Suppose G2 was the first time you heard the word Toa. Would you still hate it as much as now?
Stop comparing G2 to G1. That was never the intent of the line. LEGO did what people asked, they brought Bionicle back, but as an actual LEGO theme, not just a flimsy reason to get Greg Farshtey and Christian Faber writing stories again. G2 wasn't a bad theme, it was rather good, but the golden pinnacle everyone puts Bionicle G1 up on is so incredibly high that LEGO will never ever be able to reach it in a million years because the fan expectations are far greater than what they could actually achieve or did achieve with G1. I'd go into why G1 isn't as glorious as you remember it being but that'd not only be really off-topic, it'd make a lot of people really angry, and I've been the devil's advocate here for long enough.
If you skipped straight to the end, hi! how's your day? also, the summary of the whole post since that's why you're here: Bionicle G2 isn't terrible, and the whole community collectively drove a stake through it because it didn't satisfy our unrealistic demands. And now we're asking LEGO to do it again.