Because TV aerials are banded for the area they are used in. Digital TV uses more of the available frequencies so ITV could not be outside the original band so you could receive a weaker signal.
The aerials are 'tuned' for the specific band so they receive the strongest signals for the programmes for your area. Its all to do with the length of the receiving element on the aerial being the correct length for the wavelength of the signal you want to receive.
I could be wrong as I often am
