This is a question that i have never seen befor so i may give some resonse why i think he might have done it ,
1.The "Life Force" quirk may be reaching it's end or the possibly that it may end soon is big Problem for all for one so he is planing to get a younger body like Orochimaru or a better quirk to keep him young then "Live Force"
2.I dont know where it comes from but in the wiki it says that "All For One's body cannot properly contain Quirks of newer generations," so he might be afraid he might not be able to gain more power anymore in the future