The powers that be at Stack Exchange decided to treat comment deletion as equivalent to voting on comments. Given that those two actions are nearly always mutually exclusive (you can delete but not upvote your own comment, while you can upvote but not delete other users' comment), the reason for defining this equivalence is best known only to them. More about it on this meta.SE post and other meta posts linked from there.
One good reason mentioned there, though not entirely convincing (to me anyway), is to prevent Denial of Service (DoS) attacks on the Stack Exchange server. However, the number of comments posted is already rate limited, and by definition, the number of comment deletions is limited by the number of comments posted. Hence, this attack requires advance preparation by posting lots of comments for several months, and then deleting them all at once (using an automated script).