The following is taken from the Ofcom Technology research Programme (2005). A link to the whole document is given at the end of this note.
Only a relatively small section of the radio spectrum has the ideal properties for many of the wireless communication applications. There are many technical factors determining the ideal frequency for a particular application. Two significant factors for wireless communications applications are the range of transmissions and the bandwidth of those transmissions. There is a relatively small band of the spectrum where both high data rate and long range requirements can be supported, and hence this area of spectrum, termed the sweetspot, is in high demand. Other factors such as the practical antenna size and device costs for manufacture also influence the choice of this frequency range.