Why can't the SMS received by Motorola V3 mobile phone be directly stored in the SIM card?

I remember that my friend's V3 mobile phone doesn't have an external card, so all mobile phone messages can only be stored in the phone's own memory. The memory of this phone is only 5M, which is very small. You can download five or six softwares and store some pictures, songs and so on.

the p>SIM card can only store the information in your phone book.