I don't find that surprising at all, achievement stats don't take owned copies vs played copies into account. And many people probably played the game on GOG version too...

So generally speaking, I think the number is more like 10% to 20%

As for why.. well there could be plenty reasons. The start of the game is not very immersive, the characters not hugely interesting.. and the end game is very meh. And some of the puzzles are HUGELY frustrating.

There is not 1 singular reason I am sure, but every single annoying thing makes more people stop playing. And Cyseal.. is not a great start.