Because they say the second is worse, and the criticisms they give have to do with the story. This implies they think the story in the first is better.
Who is this 'they' you keep speaking of? I might not have been here for very long but so far from all the lurking and interactions I've had with the people here, the consensus is that CoCII's writing
is better than the first but fails to deliver the same kind of impact the older game had. When talk of praising the old game in favor of the newer one in terms of story, it's usually in the context that it manages to deliver exactly what it says on the tin despite it's lower writing quality and simplicity.
Corruption of Champions has the MC be a Champion from another realm thrown into the demonically-corrupted land of Mareth and the simple plot allows you to have free reign over most of your choices as well making a lot of things have verisimilitude. Foodstuffs that transform you can also transform others, people actually react differently to you at different levels of corruption with some companions even leaving you when your corruption is too high, the goddess of Mareth not doing anything makes sense since it's established that she has been extremely weakened.
Corruptions of Champions 2 has the MC be an average Joe that is almost literally roped in by the walking big-tittied plot device that is Cait. This in itself would not be an issue if weren't for the fact that you're put in the backseat most of the time and the fact that the honestly-interesting-and-rich-plot more often than not railroads you very hard on top of having issues that make suspension of disbelief harder. It's established that only you can transform from the normal foodstuffs that the people eat yet no one really bats an eye when you transform into some odd abomination of multiple furry traits. Potions don't count since they
do have an effect on the locals such as giving certain items to your companions to make them milky or more feminine. Corruption has little to no impact on your actions nor on others as apparently all it does is make you look evil and, again, no one really gives a flying fuck about it either. The lore has stated that the demons have invaded the land once before but apparently the gods aren't really doing much of anything and there are
multiple gods of multiple faiths.
Sure, when you simplify anything, it will look the same. You do play as a nobody in both games but who that nobody is makes a world of a difference. The plot points are not the same. Even the setting isn't the same as one is an already corrupted land and the other is a land trying ward off a second invasion.
I could go on but I think this thread speaks for itself. Now, this could probably be just me being rather annoyed when people just throw the word 'story' around since that has implications about writing and plot quality and you could just be referring to something else. If that's the case, I apologise for going on this long-winded response.
TL;DR: Please use proper terms when criticising something and take account of the context in which people say things. The two games are
not the same, the writers made sure of that.