While the other answer is correct, I'd like to add a bit more detail on why it lowers both defenses.
Physical Defense- when you're in combat, going closer to your target tends to make it easier for them to strike you. While kicks would be less effective at a close range, you can't deny that when you're in close quarters with your opponent during combat it's much easier for them to land strikes fully (therefore hurting more than, lets say, if a punch grazed your opponent). It is also harder to block strikes when you're attacking the opponent.
"Mental" (special) Defense- this certainly isn't as easy to justify as physical defense, but I have a theory. Generally, when you're fighting someone at a close distance, you tend to be completely focused on attacking the opponent as opposed to blocking strikes. You're a bit more prone to letting your mentality get in your way of protecting yourself, like dropping your hands to try to fire off punches faster. Again, this is just a theory.
Hope this helps! :)