Not every coin can be added as base currency.
Mostly due to low popularity of other base currencies currently and high costs in maintaining different markets. Although many don’t support multisig transactions either or don’t have devs familiar with them posting pull requests.
In future devs have ideas to remove base currencies in general and move to BSQ bonded off-chain trading protocol.
Currently it really don’t make any sense in adding more base currencies, even those that were added before like Litecoin and Dash are removed now, since they weren’t used much.