What unlicensed frequencies can (or should) be used for proprietary wireless communication?
Why is that noone uses other frequencies?


The radio spectrum is crammed full of users, some licensed, some military and there are a few spaces left over that unlicensed users are permitted to use. Here's what the US looks like: -



If you want a more detailed view type in the link shown on the picture. You can also find details on line, for each specific band how you can use that band. For instance, maximum power, duty cycle etc..

Here's a little snap shot of more detail: -



And for the UK there is this chart: -