I'd like to think of it as a difference between the chinese and american writing metas, because I've primarily seen transmigration (I wake up as Legolas) only in Chinese stories, and self-inserts (I visit LOTR as myself) in primarily western stories.

Also, I've noted that transmigration genre tends to be original stories and use original worlds (though often parody settings of stereotypical settings like your average inner court/harem drama, or your average Qiong Yao soap opera) and almost never appear in fanfic works. I guess people just don't want to *be* Legolas (or Arwen, or whatever).