Religious reasons. Duh.
Would make sense. Possibly worries that it all sounded too much like 'evil black magic' or something. Bah.
I think that one of the things that I like so much about the inclusion of Kas etc being part of a person's soul reflects how in ancient Egyptian mythogy, the soul was made up of multiple parts - the Ba, the Ka, the Ren, the Sheut, and the Ib, with the body itself being the Khat. The Sheut is the person's shadow, the Ib is the heart (the seat of will and emotion), while the Ren is their name, the Ba their personality, and Ka the 'vital spark of life' (in a sense the series got Ba and Ka the wrong way round). Yami Yugi seems to be Atem's Sheut combined with some of his Ba, while his Ren is the seal on Zorc. Yugi is the rest of Atem's reincarnation, the other half of his soul. Evidence for 'Yami Yugi' being Atem's Sheut is how when he transforms in the series, especially noticeable in season 0, he emerges as a shadowed form. However, once the Ceremonial Duel is complete, and he's become complete again as Atem, when he 'possesses' Yugi in Dark Side of Dimensions, he emerges in a light, firey form, symbolizing how he's no longer just a shadow. The question you can then ask is whether Atem is in the afterlife within Yugi's soul (judging how people themselves can create their own dimensions in the film), or whether he's become as a separate version of the soul, his own being in a way. I vote the former, as Yugi becomes more like Atem as he grows up. His 'life' has moved on, able to put behind his life as Atem and able to be Yugi. Through 'passing on' he's able to fully join with the rest of his soul that is Yugi.
These are just my thoughts. Like I said, I'm a bit of a geek for this sort of thing. Sorry for the bit of side-tracking. I just feel that subtle and important details of the series get missed out more, or make less sense (along with the Shadow Realm stuff) following the kiddifying of it, together with the increased focus on the card game aspect in the Duel Monsters series. At least that managed to get brought into a more interesting angle with the spirit duels stuff. I think that it's one of my big bug-bears with the dub really. It just doesn't make as much sense, and it loses some of the main charms of the original. Plus Atem sounds waaaay too old, sorry Dan Green.
[ I also think that the anme missed a few nice points about how Bobassa/Hasan is actually the light counterpart to Zorc, and how it seems to be that the spirit of Bakura's ring is a mix of Aknadin/Akhenaden, the thief King (that Bakura is, like Yugi, a partial reincarnation of it seems), and Zorc, with Akhenaden's mummy present there with Yami-Bakura and Yami Yugi during the Shadow RPG, with that taking place in the museum, behind the tablet. ]