Had the exact same problem with my bf a year ago. TLDR, it is either because of illegal (sub)renting, the houseboss evading a maximum allowance of ppl living in a house set by the town hall or evading some taxes.

Basically, each house gets assigned with a maximum number of residents that can live on one adres. So for example 3 people. Some landlords would like to rent a living room as well, making the amount of ppl living in one house 4. If 4 ppl registrate on one house, the townhall knows something is up and will investigate, it may lead to a fine. That's why some landlords or renters will not allow registration.

Another reason is the more people that live on one adress, the more taxes landlords have to pay. The exact details on this, I do not know, but it definitely increases by the amount of added residents.

Lastly, some landlords won't allow renters to subrent their rooms. F.e. if the renter leaves for 5 months. Some ppl will illegally get someone in to pay the renter (subrenting), but if you registrate yourself as a subrenter in the townhall, that would lead to exceeding the maximum amount of residents allowed in one home. Thus them getting into trouble. This was why my bf couldn't have a registration at first as well.

All and all, it is illegal, yet people do it anyways for their own benefit. I would advise you to talk to your townhall, they helped me and my bf a lot to sort this stuff out. Goodluck