I'll be as honest and as thorough as possible. It's because:

• Game clearly wasn't as polished as the development team wanted it to be, but ubisoft the publishers know that allows them to squeeze a bit more money from DLCs. So content they would have released on day 1 was moved to dlcs down the line.
• The main campaign ends spontaneously at a stupid point in the game
• The main campaign presents two approaches in it's second region but they are not balanced
• It had glitches upon it's release.
• political reasons and woke. Anno 1800 already "went too woke" in people's eyes by trying to push a vague sense of a society more equitable than it really was in the 1800s - I don't mind either way because while that's a good argument, the game is in a fictional world not perfectly resembling our own, so the creators have that freedom.
• How did anno 117 do this? Well it has a female emperor, her daughter does trade which should be illegal for the nobility, and women. And it glasses over slavery a lot.
• compared to Anni 1800 which has 4 years of DLCs (12 large DLCs), anno 117 had less content in release. It already feels like it is nearing 1800 is size, to me, and also that's a stupid reason to dislike it
• it is needlessly more demanding for your PC. 1800 was already fun to look at. There are diminishing returns on the graphics.

Some stuff that I have a problem with but haven't seen criticised:

• the pirates are way too tough to handle and make the game not fun
• economy is honestly badly balanced. I'm an experienced anno player and playing it on tough settings makes it really horrible to get big things like military excursions done. I'm pretty sure that only people on easy (aka default) settings are conquering other islands and conquering pirates with ease.
• adding to the economy: game gives you city walls but there is never any reason to build them, they are less effective.

I think the game tries too hard to be polished and ends up being clunky as a consequence. Moving troops, moving ships, placing buildings (especially the special ones like canals and aqueducts) juuuust doesn't work.