Police scanner - Aviation use?

I have a police scanner and I have an antenna from C. Crane Company. My problem is that I have it programmed mainly for usage for aviation. I live 100 miles to the east away from Minneapolis/St. Paul International Airport and I can hear aircraft transmitting about 50 miles west of MSP airport. I have the tower and ground frequencies programmed in, but I cannot hear them. So why can I hear aircraft about 50 to the west of Minneapolis/St. Paul International airport, but not directly from the airport? Thanks.