Themes aren't the same. Dimmu Borgir typically writes about armageddon, anti-christian/'satanism' and lyrics more common of black metal etc where Cradle has always embraced more gothic themes such as vampires, romance, fairytales, Elizabeth Bathory, poetry and such. Why does naked women in album artwork indicate a copy, it is very very common among males is it not? The way in which the women are portrayed is also very different, Cradle does it in a way that's more dramatic and would compliment beauty as opposed to just senseless nudity.
Musically they are different as well. Dimmu Borgir comes from a black metal background and guitar work is very different to Cradle's Iron Maiden influenced guitar harmonies albeit neither band is guitar-driven. Cradle's 'atmosphere' is also different, it works to produce a more gothic mood which I've never really found in Dimmu's music at all. Even though Barker was in both bands, the drum style is different from both bands with Dimmu being centered towards more skilled drummers in Barker, Hellhammer, Killerich, Laureano where Cradle isn't particularily concered about that.
You're simply drawing similarites from very general things. I feel that these comparisons are really only derived from both bands highly successful and appealing to a certain audience. Dimmu Borgir is not even a gothic band, even if they appeal to a so-called goth crowd. The comparisons are bogus unless you are relatively new to either band, but if you've been into both bands for years like I have you'll be able to see the frequent 'similarities' aren't as valid as people would like to think they are.
The reason why someone would enjoy CoF and DB while not being able to stand black metal is not because they have some symphonic or melodic components but because they are more accessible than black metal bands. They are also at the forefront of extreme metal as opposed to more obscure and underground acts, along with bands like Opeth, Children of Bodom, Arch Enemy - those bands are accessible and therefore more popular to a general audience. Black metal's raw underproduced production qualities, and harsher vocals is enough to turn away the casual metal fan.