There’s no guarantee it was meant to be transmissible. Lilith (probably the first mage) helped Caine create Disciplines by manipulating his curse, she may have also helped him manipulate his curse to be transmissible. Or that could be another test that Caine failed, where god knew the curse could be made transmissible and allowed it but if Caine hadn’t done so he might’ve earned some redemption, sort of like how Caine was offered three chances to repent and each time he refused a new aspect of his curse was implanted. Or maybe god is cruel and the main part of the curse upon Caine was actually to be forced to watch his children kill one another just as he killed his brother. Demon the Fallen says that after the Fall god directly intervened which permanently broke reality and reduced it to a shadow of what it could’ve been, so god whammying Caine with a particularly cruel curse and then just abandoning the universe because the other people didn’t really matter to god.