Many reasons, their choice of material, how they market high end specs with mid range build quality (phones, laptops and TVs)... Their dedication to Android, their low end phones are so bad all around, specs and hardware, and they are usually hideous.
Keep in mind this is just my opinion, and I'm very passionate about technology and design as a whole. Some people might not agree, but I respect that.
On the other hand, I have really enjoyed what Nokia has been doing with the Lumia line. Although I've never owned many Nokia devices in the past, one of my favorite old phones of theirs was the music xpress (5500 I think it was?).