That's because altcoins are still relatively new, and more and more exchange are popping up which allows altcoins to fiat trading; like Vault of satoshi, prelude.io, and Cryptsy plans on allowing that soon.
Some of the altcoins have been around since 2010. How many exchanges are allowing them? And regarding the
"soon to be allowed in exchanges such as Vault of satoshi, prelude.io, and Cryptsy" argument.... It is all BS. They will never be allowed in any of the major exchanges. Even in 2013, a lot of people were posting rumors that Mt Gox will soon accept altcoins. Nothing happened. However, the posters benefited when the value of altcoins rose as a result of the rumors.