Well, firstly, everyone has long been accustomed to the term cryptocurrency. Secondly, we can't call them digital money, as this term means not only cryptocurrency but also other means of payment (WebMoney, for example). And also, by the word "cryptography" is meant confidentiality, transparency and authentication - all those properties that are exactly inherent in bitcoin and altcoins.
When we say crypto currencies, it means that we are talking about the digital currencies which include both bitcoins and all the altcoins / tokens etc. This does not include the digital money like Webmoney / skrill etc.