Well personally, the period always crashed my libido. Most desire right before it, then boom, crash, just sleepy, hungry, or migraine when it started. So my answer would be “because I didn’t want to”.

Anatomically, the cervix is lowest during the menstruation, so you are more likely to bump it, but I am not sure how much of a difference that makes.

There is no safety reason not to have sex when you are on the period though - so if you both want to, go for it.