The reason is obvious.
ANY licensed person living in a household, can have access to drive your vehicle at ANY time. Even though you will be the only one driving, at ANY time, someone could use your vehicle, be it to drive to the store, or use for any reason, thus, they need to "rate" each driver.
It could be that the other insurance company had that information and you was not aware of it, thus higher premiums.
After you give them this information, then see if in fact you are getting a better deal or not. If you fail to give this information, and IF, one of these people drives and then has an accident, then your insurance company can DENY payment on a claim.