Immersion can be part of it but the ASIs can break immersion. Maybe I want to play a completely ugly and unable to be convincing tiefling. Therefore I would want a negative in charisma to match but that +2 to charisma stops me from having the negative I desire. Tieflings are considered innately charismatic but I want to play one that isn't, or a person might want to play a weak orc, or dumb gnome. Races in dnd, while being more like species, also have variance like humans where they can have different strengths and weaknesses. ASIs reflect the average member, but most people are not playing average characters. If I go back, the +2 will always make him average at being convincing, the gnome will always be average intelligence, and the orc will always be average strength even if they shouldn't. The loss of variance by locking ASIs is immersion breaking for me and locks off character concepts matching in RP and mechanics.
