Why is only a limited of assets/currencies/tokens available?

For currencies/tokens to be featured, there needs to be a reliable, trustworthy token/IOU issuer for the given currency. 

You can easily add currencies by tapping the Add (token) button on the home screen. This is a curated list with trustworthy assets. You can add any other asset using a tool like xrptoolkit.com / xrpl.services and sign with XUMM.