How can I explain to Mom why it's hard for us to visit more often?

My mother lives in another state about 300 miles away. She doesn't understand the difficulties of traveling such a long distance to visit her, especially since my husband and I work long hours during the week. I call her every Sunday, and every Sunday she tries to make me feel guilty for not bringing her granddaughter (age 4) to visit. Mom can't drive, so her visiting us requires my sister to make the long drive (sister works during the week too, so she has the same complaint about the long drive). Mom won't accept the fact that we have a life in another state, and we're not going to give up everything we've worked so hard for just to move closer to her. How can I get Mom to understand that we're not being mean, we just don't have the time to visit her as much as she'd like? (It hurts her feelings when I try to explain that we need our weekends and holidays as a family at home, and that we can't be taking vacation time from work just to make a 3-day weekend to visit her.) Help!