Because it's a good earphone. Personal preference always plays a big roll though. I'm big on finding faults of a product and highlighting weaknesses. It's the only decent way to absolutely define one product as being worse than another. I have come to see faults with products less on a preferential role and more on a capability role as in it either can or can not perform the task. In this sense, a product can recreate sound appropriately, be it in a colored way, or it has specific limitations of recreation of sound that prevents it from doing certain things. It's good to see a product for what it can and can not do absolutely. This is raw, unbiased capability, and pretty much the only unbiased method to rank products. Stepping beyond this, all we have is personal preference, and that starts becoming a matter of opinion. When a product has minimal faults, it is a good product regardless of how it sounds. Coloration is desirable by most people.
The IE8 offers a lot. There are a few faults, but I haven't listened to an earphone that doesn't have any. The driver is sluggish enough at high frequencies to blur together smaller details. The sensitivity is there, but the bass geared driver has inherent limitations. It's not bad really, but it isn't perfect either. The sound stage is massive in sense of distancing. It's great for that big sound, but it means close stuff sounds far away. There's basically no sense of proximity and subsequently layering and space between sounds in a 3-dimensional sense. Those are the only two "serious" faults of the earphone. Everything else is fixable or a matter of personal preference.
You own the CK10 and value that highly. The CK10 is another earphone that offers a lot. The micro detail in the high frequencies is basically unmatched. It has an excellent sense of realism of sound. It does a lot of things right. However, there are also limitations of that earphone too. The sound stage is a little limited in being able to place a sound in a pinpoint spot in a virtual space and depth of field is a little limited. Bass on the CK10 can be a little muddlied in the lower frequencies too. Frequencies response is fixable, and everything else is a matter of personal preference.
I can say something like the CK10 is one of the better devices out there, but the IE8 is a top level product too. They're geared very different from each other, and certain "likable" characteristics of each will depend upon personal preference. I liked the IE8 when I owned it. I still miss it a little bit, but I also understand my preferences well and know I tend to favor other options a little more, not because they are holistically better but because they fit my preferences better.