Why aren't all fields showing up in dropdown box when I am changing the mapping for lead to account?

Why aren't all fields showing up in dropdown box when I am changing the mapping for lead to account?

Some info is lost when I am converting Leads to Accounts or that info just goes into Contacts but I need it to stay in Accounts. I tried to change the mapping of those certain fields but in the dropdown boxes, the corresponding fields are not even listed as an option for me to chose from. For example: trying to map "Skype ID" field in Leads to "Skype ID" field that I created in Accounts. I go to the dropdown box and there is nothing there for me to chose from. So when I convert the lead to an Account, I just lose that information? Please help.