Sometimes, the answer to why someone doesn't like something just boils down to, "I don't like it," and it may be difficult to quantify specific reasons. There is definitely some of that going on. But here is what I can definitely say:
As with a lot of progressive metal, there is a lot that is so complex that it can feel a-tonal, a-rhythmic, and/or not melodic. When you take the time to understand it in context of what the band is doing, the complexity often falls away and it just works. With Haken, I find all of those elements to be present, and it does NOT work for me. To compound the problem, I find the vocals to be especially grating, which REALLY distracts me from the music and from possibly getting the music. Not sure whether I worded that in a way that makes sense, but that's my best effort.